Last summer, Mark Arduini was unemployed and struggling to motivate himself to fill out job applications. So he went to a local card shop, bought a box of 24 unopened packs of 1987 Topps and bribed himself: He could open one for each application completed.

As he was flipping through one pack, he came across card No. 500, Don Mattingly. On the back, under Mattingly's career stats and above a line of baseball trivia, Arduini read the one-line Mattingly bio. It was a jaw-dropping scoop: "Don's birth certificate states he was born in 1962, not 1961 as shown in most baseball records."

The birth date at the bottom of the card is also 4-20-62, and if you look closely, you can see that the "62" is slightly off-line. The first draft of the card, it appears, had used the '61 common to most baseball records. One imagines a "Stop the presses!" moment at the Topps factory, as the correct year was apparently fixed by hand.

Arduini went online to confirm that most baseball records had since been corrected. But they hadn't. Baseball-Reference.com: April 20, 1961. Wikipedia: April 20, 1961. MLB.com: 4/20/1961. "Everywhere I looked, the birth date still -- 32 years later -- hadn't been fixed," Arduini told us. "So I checked back in with Topps, and guess what: On Mattingly's 1988 card, his date of birth was listed as 4-20-61. This fun fact, Mattingly's 1962 birth date, only appears in two places: his 1987 Topps baseball card and his birth certificate. Now, I've never seen Mattingly's birth certificate, but Topps has; they said so right there on the card."

So Mark hired us to solve this puzzle. One route would be tracking down Don Mattingly's birth certificate, but in Indiana, birth certificates are presently considered closed records, available only to people who can provide "proof of relationship or direct interest to the person named on the record." We then tried to go to the source: Mattingly. We asked the Marlins' press department, who happily pledged to find out from Don himself. And we asked Mattingly's agent, who also chipperly promised to get the truth straight from Don.

And then neither party was heard from again. We started to wonder whether Mattingly, and the powers that be, wanted to keep something a secret.

So we had to take a different route: We had to dive into the baseball card industry circa 1987, when it was transitioning from making disposable bike-spoke smackers to producing the higher-quality, get-rich collectibles. We had to track down the uncredited writer of a single sentence written on the back of a piece of cardboard more than three decades ago so that we could figure out whether the author of "Don's birth certificate states he was born in 1962, not 1961 as shown in most baseball records" was a crank or a whistleblower. And that's what we did.

He wasn't a crank.

The Hit Man

In the late 1980s, a few years after Donruss and Fleer broke Topps' monopoly on the baseball card industry but a few years before the card companies began to seed their sets with rare inserts and subsets, there were two types of baseball cards worth more than a buck or two: the ones that were really old and the rookie cards of star players. Even the latter usually took time and patience for investors, as the player's stardom and stature needed time to grow. It was hard to open a pack and get rich -- even rich by kid standards.

Don Mattingly was the biggest baseball star of the early junk wax era, and because he had gotten so good so young, his early cards were both valuable and attainable. In 1984, when he was 23 (according to most baseball records), he won the batting title. He had power, smooth defense, charisma and pinstripes on his uniform. He was prophesied to be the next great Yankees icon, picking up the lineage from Ruth to Gehrig to DiMaggio to Mantle, which had seemingly reached its end with Mantle's 1968 retirement. In 1985, the supposedly 24-year-old Mattingly won the MVP award, won his first Gold Glove Award and drove in 145 runs, the most by any hitter in nearly a decade. In 1986, he finished second in MVP voting, and by 1987, his Donruss rookie card was going for $100. It was so precious that it inspired counterfeiters, one of whom was himself such a precocious talent -- a 13-year-old Florida boy who printed 1,000 at a local copy shop and then sold them for around $40 apiece to gullible dealers -- that his scam became a national story, further elevating the Mattingly card hype. "That's why I make an argument for Don Mattingly to be in the Hall of Fame," a former Upper Deck executive told Dave Jamieson, author of the book "Mint Condition: How Baseball Cards Became an American Obsession." "He's the one that bailed out the card industry in 1987."

Baseball players have lied about their ages forever -- even Hall of Famers -- but they do it to make themselves younger. A 20-year-old putting up good numbers against 18-year-olds isn't a prospect; a 16-year-old doing the same is. Indeed, when there were references to Mattingly's age during the first half of his career, they snarkily suggested he must be older than he was listed: "Somebody find the birth certificate. Don Mattingly can't be only 24 years old," the South Florida Sun-Sentinel wrote in the spring of 1986, right before Mattingly's official 25th birthday. Writing about him in the spring of 1989, just before his official 28th birthday, Peter Gammons called it "hard to believe" that Mattingly is "just 27 years old." Harder still to have believed he was even younger.

But that's what 1987 Topps charged Mattingly with. Donnie Baseball was already a star, but his early career, reassessed under the 1962 birth date, begins to look legendary:

• His .345 batting average would have been the highest by any 22-year-old since Stan Musial (in 1943) and Ted Williams (in 1941).

• His .349/.444/.488 line during his first year of pro ball would have come when he was nine months shy of his 18th birthday, a full year younger than his draft class.

• When he hit .463 over his four-year high school baseball career, and reportedly tied the all-time national record for career RBIs, it would have come while he was actually the equivalent of an eighth-through-11th-grader.

• When he was named all-city in basketball as a junior -- in Indiana, a state known a little bit for its high school basketball players -- he would have actually been the same age as most sophomores.

We do know that Mattingly had been publicly identified with a 1961 birth date even before he joined the big league club and got famous.

• In September 1980, when he was still in the minors, the New York Times called him 19 years old; that puts his birth in 1961.

• When he was in his first professional season, his manager filed a scouting report on him, describing him as 18, which would imply the 1961 birthday.

• Right after his high school graduation, he was featured in Sports Illustrated's Faces in the Crowd. It said he was 18, reflecting the 1961 birth date.

So if he were lying, he was doing it at least as far back as then.

Why would a player conceivably lie to make himself older? That's hard to reckon. One initial theory was to make it possible to get married. Mattingly was 18 -- by the 1961 birth date -- when he got married in the summer after high school. Maybe, we guessed, he would have needed to for a marriage license application. But that theory was weak: His wife was only 17 at the time, a grade behind him, according to news reports from Mattingly's early career. She actually left school to follow him. So it's clear the couple didn't need to be 18 to be officially wed. Perhaps a very young Mattingly would have needed to fudge a birth date to play a level up in youth leagues, so he could play against competition that would challenge him? That makes ... some sense? Maybe?

But the record seemed to be pointing toward the "written by a crank" explanation. Yes, we found a second reference to a 1962 birth year -- in "The New Bill James Historical Baseball Abstract" (2001) -- but James told us those birth dates were all drawn from "standard sources," suggesting this was merely a typographical error. And we found a confusing discrepancy in the 1988 edition of the Baseball Encyclopedia, where his birth year is listed as 1961, but his birth date is listed as April 21, not April 20. But the next edition had his birth back on April 20 (1961).

Everything else but the card was consistent. Until, in the process of trying to read every article that mentioned Don Mattingly and a birthday, we discovered an Associated Press article from April 21, 1987, a few months after Mattingly's 1987 card back would have been written:

Mattingly's second homer of the season, in the first inning, came on his birthday. Exactly which birthday is subject to debate; he says he is 25 years old, the Yankees say he is 26.

The story goes on to say that Mattingly showed the Yankees his birth certificate to prove it.

Whaaaaaaat ...

Cardboard Literature

In 1987, you wouldn't go to the back of a baseball card to discover a player's deepest secrets. You might not go to the back of a card for the words at all.

Donruss' card backs used simple three-dot lists of career highlights (e.g. "Joined Lou Gehrig as the only Yankees to achieve 200 hits three straight seasons ... AL MVP in '85"). Fleer published low-fi heat maps of hitters' strengths in the strike zone. Topps used its card back space to carry one more statistical column than the other two. It included a piece of baseball trivia unrelated to the player on the card. And it offered a one-line, first-name-only biographical detail that was almost always innocuous, to the point of self-parody:

• Ken [Griffey] and his wife are the parents of two sons.

• Kevin [Mitchell] devotes time to spend with NYC youths.

• Mike [Greenwell] was voted MVP of his Little League team.

• Bob [Brenly] derives enjoyment from college basketball.

These are the sorts of details you might pull from team media guides -- and, indeed, many of the Topps bios at the time probably were. To that end, we checked with the Yankees to see whether their media guides in 1986 or 1987 could have been the source of the 1987 Mattingly card back. Michael Margolis, director of public communications, said the media guides listed his birth year as 1961: "I can tell you according to the 1986 media guide he enjoys racquetball. No mention of the birth certificate."

In retrospect, it's odd how little craft was put into those bios. "Baseball cards are the only product on a typical candy rack to set forth baseball statistics. They are, in other words, an education in baseball," wrote the judge who ruled against Topps' monopoly in 1980. But it's much more common to hear adult baseball fans say they learned math from the stats on the backs of cards than to hear they learned to read from the backs of cards. For decades, a baseball card back was one of the most read pieces of literature in the sport, especially for young fans, and yet they revealed almost no ambition.

By 1987, that was about to change. In 1988, Score became the fourth major card manufacturer, and its card back bios were full paragraphs, crammed with detail and expertise. "When Score first came out, I can remember their promotional stuff saying that card backs would be written by a Sports Illustrated writer," says Bruce Herman, who would later be Topps' head writer. Then in 1989, Upper Deck emerged as a high-quality competitor, and by the early 1990s, Topps and all the other brands established their own premium lines to go with their main sets. Then card companies added subsets, parallels and inserts, and each card had to have a back.

Phil Carter was the director of sports for Topps at the time. "At one point we had 25 cards of Cal Ripken in one year across all different products -- how many different things can I write about Cal Ripken that hadn't been said before?" He knew it would look cheap, pathetic even, to have the same one-line bio on 25 cards, so he turned the card back writing over to Herman, who would become perhaps the most read, most prolific baseball writer of the 1990s and 2000s. He has written, by his estimate, a quarter-million card backs, comprising 10 million words -- none of them credited. His card backs are often funny, often brilliant, often statistically advanced and always novel. In a dozen words he could perfectly, precisely capture a player's essence -- "[Will Clark] has called George Brett 'the only baseball player I admired'" -- and then do it again and again in new ways across a player's 15-year career.

But those literary card backs were just over the horizon. In 1987, Topps had to write only one card back per player per year, nobody expected these cards to be premium, competition for market share was still minimal and one of the primary goals of a card back bio was to not annoy the player (as when the wrong wife -- the ex-wife, not the new wife -- was named, and the player yelled at Topps for it). Phil Carter doesn't remember that Mattingly card back, but he remembers who would have written it. "When you started telling me about a birth date change, a smile came on my face," he says. "It would be reminiscent of one of my guys -- a gentleman named Bill Haber, in Brooklyn."

'Baseball's One-Man FBI'

Bill Haber wrote the card. Probably. From 1967 through the late 1980s, Haber wrote pretty much all of the cards.

Bill Haber is a legend, if not for his uncredited work on the cards. In 1971, he and 15 other baseball scholars founded the Society for American Baseball Research, the "SABR" from which sabermetrics would later take its name. "A very strong argument can be made that [Tom] Shea and Haber were baseball's greatest biographical researchers," a SABR journal from 2000 said. He was "baseball's one-man FBI," in the words of New York Daily News writer Bill Madden. He was a superstar on SABR's biographical committee, which has unearthed and corrected thousands of names, birth dates, death dates and other now-official (and now-accurate) records of historical ballplayers.

Haber's superpowers were persistence, creativity and the United States Postal Service. He would send letters to funeral homes and cemeteries, to ballplayers' descendants, to newspaper archivists and to public agencies -- along with the fees demanded for broad public records searches -- hunting for clues. Sometimes, he just needed to pin down a cause of death. Other times, it was up to him to unearth, nearly from scratch, the biographical data of players who might have appeared only in a game or two, perhaps under only a surname, perhaps under a fake surname. As Madden recounted in his tribute to Haber: "Most recently he solved a 100-year-old mystery involving a player listed in the record books only as John McGraw (not the legendary N.Y. Giants manager) who pitched in one game for Brooklyn of the Federal League in 1914. The only other information was that he was born in 1890. It took years of painstaking research for Haber to determine McGraw's real name was Roy Hoar which he changed to Heir and then played professional ball under the name McGraw so as not to lose his amateur status at Carnegie Tech. The final clues to Hoar's true identity were supplied by his two sons, whom Haber tracked down in California."

Everything he found would go to SABR's biographical committee, which would publish the new finds in its journal. The Baseball Encyclopedia, Retrosheet, the Baseball Almanac and other references would then make the corresponding changes in their records.

Haber also kept his own records. He had multiple index cards for every player who had ever played, Carter remembers: one for date of birth, one for cause of death, one for middle name and so on, all filed in long rows in his Brooklyn basement.

"I know that a lot of it stemmed from his interest in baseball cards," says Bill Carle, who runs the SABR biographical committee. "He had baseball cards back from the teens, and he was interested in making sure that the information was accurate on the guys he had baseball cards for. I think he probably originally got into it for his own curiosity, and then later on he knew that this served a larger purpose."

What, exactly, was that larger purpose?

"Well, I want to see accurate information," Carle says. "And it bothers me when something isn't accurate. If you look in the Baseball Encyclopedia and looked up Babe Ruth's 1927 and it said he hit 59 home runs, you'd go, 'How could they make this mistake? It's 60!' Well, I do the same thing with birth dates and death dates. We've had players, you look at it and discover they were supposedly 14 when they made their debut. I look at that and say that's wrong." Haber -- by all accounts a friendly man who loved to work in solitude -- agreed.

While Haber was writing those letters, enclosing $40 checks for public records searches, he was also in charge of the backs of Topps baseball cards. Most of his emphasis was on the stat tables, which he calculated and created himself, before computers. He'd disappear for the first few weeks of the offseason, make all the tables and deliver them to Topps for printing. He wrote the card back bios too, but he relied on official media guide information and generally kept them anodyne. It was safer to do it that way. The first rule was to do no harm: Annoy no ballplayers.

Haber died in 1995. His son, Marty Haber -- for a time locally famous as the Brooklyn Cyclones' on-field emcee Party Marty -- once boasted that "six people filled his position at Topps."

Haber's research emphasis was on early ballplayers, and it's unlikely he would have sleuthed for Mattingly's birth certificate as he had for Roy Hoar's. But what we do know about Haber is that he likely would have taken more interest in Mattingly's date of birth than the average baseball fan, writer or scholar would have. Birth dates were his thing. He would have been very interested in correcting the record if it were wrong and in clarifying a discrepancy if there were one. His work writing card backs for Topps might not have been as fun as his hobby of digging up historical inaccuracies for SABR, but with Don Mattingly's birth date, the two would have bumped against each other. Haber got a chance to correct a record.

But then, why would the 1988 Topps card have reverted to 1961 for his birth year? And why was the official record never changed? Was the card right? At this stage in the research, there were three guesses:

Carle, Haber's old partner at the biographical committee, guessed that Haber just made an error. It happens. Notes get transcribed wrong. Carle had an Indiana baseball expert on his committee -- who recently died -- and Carle figured if Mattingly's birthday had been wrong all this time, that guy would have definitely found and flagged it. So that was Carle's guess: "1962" was an error, made once.

Carter, Haber's old boss at Topps, speculated that Haber discovered the discrepancy somehow -- "That would be his thing" -- and put what he figured was the real date on Mattingly's card. And then, Carter's speculation continued, maybe Mattingly complained about the weird blurb to a Topps rep visiting the Yankees clubhouse, or maybe Mattingly's agent complained, or maybe the Yankees did. (The Yankees' PR director at the time, Harvey Greene, doesn't recall anything about this.) And since it was not just any player but Don Mattingly, and not just any team but the Yankees, maybe Carter would have just agreed, no big deal, to go back to the 1961 year. To be clear, he doesn't remember it that way; he could just imagine it going down like that. In Carter's theory, Haber could have been right, or Haber could have been wrong.

My guess was that Haber was right. Maybe Mattingly skipped a grade in elementary school, and when he entered the draft somebody just assumed he was 18 like the other seniors, not 17, and put the wrong date in. Probably Mattingly never told anybody the wrong year at all, but one person typed it in wrong one time. And once that was in baseball's official records, it got printed over and over again, in media guides and then media articles and then baseball cards, a mutation replicating. Mattingly tried -- he even showed the Yankees his birth certificate! -- but he couldn't get it changed. Classic modern problem: Hard to correct something once it's In The System. Topps briefly had it right. And then, by 1988, Topps forgot too and reverted back to the mutation.

Fine theories. None of them is what actually happened.

What Actually Happened

It was, ironically, only when we shifted our interest from Mattingly to Haber that the mystery cracked open.

After we interviewed Bill Carle about Haber's legacy and methods, Carle hung up the phone and started his own search. Using the same methods Haber had used -- but now modernized by the Internet -- Carle found a birth notice in an Evansville, Indiana, newspaper. Mr. and Mrs. William D. Mattingly, 505 Van Dusen, had given birth to a son, Donald Arthur.

The date on that newspaper: April 1961. Don Mattingly's birth date was definitely settled.

The mystery, though, was not. Are we supposed to believe that Bill Haber, a titan of baseball research, the 1985 Don Mattingly of baseball accuracy, just made this up? It's not, after all, just a birth date on that card back, an easy typo. It's a sentence, full of certainty and citing an actual government document: "Don's birth certificate states he was born in 1962, not 1961 as shown in most baseball records."

After his death, Haber's research notes were donated to the National Baseball Hall of Fame and Museum. I wondered what they looked like and whether there was an index card where he'd written Don Mattingly's birth date, so I reached out to Cassidy Lent, the manager of references services at the Hall's Giamatti Research Center. Long ago, Haber's notes were incorporated into the library's player files. It wouldn't be as simple as, say, opening up the Haber box. As a starting point, Lent suggested sending me their Don Mattingly file.

She scanned it and sent it in a PDF, 200 pages of clippings from Mattingly's career: His first ever paycheck (for $292); the scorecard for his first professional hit (attendance 2,954); a news release from his agent hinting at his imminent retirement; and hundreds of news articles from throughout his career, including -- 126 pages in -- a pair of articles that hadn't turned up in Lexis-Nexis searches.

The first one, from 1986:

That predates the writing of the 1987 Topps. It's a reasonable guess that Haber saw it, or saw another local paper with the same odd sidebar on the same day. With his keen interest in correcting the record of misstated ballplayer birth dates, it's a reasonable guess he jotted himself a note to put the correct date on Mattingly's next card. And then, a year later, Mattingly had another birthday, and some reporter remembered the funny detail from April 1986 and asked Mattingly about it again. That produced the Associated Press article I found earlier, from 1987, when Mattingly restated his 1962 birth date, declared again that the official record was wrong and claimed he had shown the Yankees his birth certificate to prove it.

But this time, the Hall of Fame folder goes on to show, Mattingly couldn't keep a straight face through the whole news cycle. A New York Daily News article, under the headline "The Hitman Comes Clean: I'm 26," gets Mattingly to reveal the plot: A year before, he had been in a terrible slump when his birthday arrived. Eleven games into the season he was slugging .267, still looking for his first extra-base hit, and reporters wouldn't stop asking him about it: "Every day, the same question. I was tired of answering the same question, so I changed the subject. I grabbed a writer and told him the book had me one year older than I was. And that became the story for the next few days. By the time that story died, I had a couple of extra-base hits and I didn't have to answer that question anymore."

The only victims, if there were any, were the reporters who put their byline above a hoax. And also the meticulous baseball researcher in Brooklyn who spent so much of his life dedicated to producing a pristinely accurate record of Major League Baseball history. For him, Mattingly's hoax survived as a slightly embarrassing mistake -- one moment of Bill Haber's remarkable career slightly askew, just like the pasted-on "62" on the back of that 1987 card.

But Mattingly's hoax is also a good story. It's a great story! Charismatic and confident Donnie Baseball was so frustrated by a slump, and so sensitive to public attention, that he created a nonsensical lie to distract us all from his struggles. And it worked. People believed it, or didn't know what to believe, while he hit three doubles the next day and went on to have another great season.

In some ways, the Don Mattingly hoax is a classic baseball story: small and quirky, worth only a brief mention on the inside pages of the day's sports section, but in time, a better and more significant tale than the news that made the cover. First it had to burrow into the soil, dormant and nearly forgotten. Decades passed, and time erased many of the original details. But other records survived, and technological progress made them accessible to a distant generation. Just enough of the truth hung on, waiting to be rediscovered and retold, until a great yarn of baseball history could be recounted. Ultimately, the record could be proved definitively -- in this case, all thanks to the one-sentence clue on the back of a now-worthless baseball card. That seems perfectly in line with what Bill Haber would have wanted.

Mark Arduini found a job doing analytics for an art museum. He has two packs left.