When it was auctioned in 1999, a document called “Laws of Base Ball” had no known author but obvious significance.

Within its pages were fundamental rules like nine men on a side and 90-foot basepaths.

But the “Laws” — and two documents inspired by it — sold for only $12,650 in a Sotheby’s auction devoted to books and manuscripts.

Now, the buyer is turning into a seller in an online auction by SCP Auctions that will begin on April 6.

And the author of “Laws” has a name: Daniel Adams, known as Doc, a significant figure in mid-19th-century baseball who has come to be viewed as a founding father of the game.

As a player for the Knickerbocker Base Ball Club, Adams developed and played the shortstop position.

“As a captain,” he told the Sporting News in 1896, “I had to employ all my rhetoric to induce attendance, and often thought it useless to continue the effort, but my love for the game, and the happy hours spent at the Elysian Fields led me to persevere. During the summer months, many of our members were out of town, thus leaving a very short playing season.”

He subsequently became president of the club as baseball was starting to become popular.

Adams’ hand in the sport’s early rule-making is not a revelation; instead, it is the physical record of his central role memorialized in the three surviving pages of his document.

“When you come across this rough draft of history, it’s compelling,” said John Thorn, Major League Baseball’s official historian. “It’s like the Dead Sea Scrolls — it is endlessly worthy of research.”

Thorn, who is serving as a consultant to SCP on the auction, added: “I didn’t know these documents existed and never imagined they would surface. They are improbable survivors.”

Adams composed the “Laws” in an upright script and strong hand. He refers to batters as “strikers,” balks as “baulks” and runs as “aces.”

In his third law, Adams wrote: “The bases must be four in number, placed at equal distances from each other, upon the four corners of a square, whose sides are respectively thirty yards. They must be so constructed as to be distinctly seen by the umpire and must cover a space equal to one square foot of surface.”

The laws were put to paper when Adams presided over a convention in 1857 that brought together 14 New York-area teams to revise existing rules.

He and two other officials from the club, William H. Grenelle and Louis Wadsworth, were delegates to the convention.

“They were presenting the Knickerbocker plan to the convention,” Thorn said.

Adams wrote his rules, which Grenelle transferred to more than a dozen pages of blue paper in a flowing, beautiful calligraphy. Grenelle’s version, which is also part of the auction, reads like a scorecard for the debates that ensued at the convention. He used a pencil to cross out words and sentences, to record changes to Adams’ proposals and to note which laws were adopted.

“It wasn’t like signing a blank check,” said Richard Hershberger, an early baseball researcher. “There were negotiations and multiple conflicting interests.”

At one point, Grenelle scratched out “nine” and replaced it with “seven” for the number of innings to a game. The convention adopted the seven-inning rule but, at Wadsworth’s urging, changed it back to nine. Another rule passed by the convention was a precursor to MLB’s Rule 21(d), which governed Commissioner Bart Giammati’s banishment of Pete Rose for betting on baseball:

“No person engaged in a match either as umpire, referee, or player,” it said, “shall be directly or indirectly interested in any bet upon the game.”

The convention was a seminal event in baseball history, Hershberger said, that was thoroughly reported by newspapers.

“So I don’t know if this document is historically important for its content,” he added, referring to the “Laws.” “But as an artifact, it is important. If I had a few thousand dollars to spare, I’d grab it.”

No such document exists at the Baseball Hall of Fame, whose pre-integration committee considered Adams for election last year. But he fell two votes short. His next chance will be in 2018.

“I didn’t know much about his background,” said Bill Francis, senior researcher at the Hall. “I work with baseball all year, but his just wasn’t a name that crossed my desk a lot. But as I did more research last year, it was like uncovering an early purveyor of something you love.”

A far more renowned writer of 19th-century baseball rules was Alexander Cartwright, who set down 20 of them in 1845. But, as Thorn said: “Cartwright is given credit for writing rules he didn’t. His plaque in the Hall of Fame says that he set the bases at 90 feet, established nine men to a game and set games at nine innings. All that happened in 1857.”

Adams’ connection to the 1857 documents — which also include match-play rules in Grenelle’s hand — has elevated SCP’s hopes that they will sell for well above $12,650.

In two recent examples, documents related to the roots of other sports sold for more than $1 million. James Naismith’s 13 rules of basketball were auctioned in 2010 for $4.3 million. But Naismith created basketball, which conferred enormous value on his two typewritten pages. Adams did not invent baseball but was an important part of its evolution.

“The rules of basketball should be valued on a different level,” said Leila Dunbar, a former Sotheby’s executive who consulted on the Naismith auction.

And in 2011, the world’s oldest known soccer rule book was sold by Sheffield FC for $1.4 million.

Unlike Sotheby’s, which underpromoted the “Laws” documents in its catalog, SCP is marketing them as “the birth certificate of the modern game” and the “Magna Carta” of baseball.

“I’ve held items of extraordinary value,” said Dan Imler, vice president and managing director of SCP, mentioning a Babe Ruth uniform that sold for $4.4 million. “But we’ve never had anything in our history that speaks like this to the creation and development of baseball. Without these documents, there might not have been a Babe Ruth.”