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.