On April 22, 1876 — 141 years ago this Saturday — the Boston Red Stockings beat the Philadelphia Athletics, 6-5, in the first game ever played in the National League of Professional Base-Ball Clubs, the outfit that would ultimately become Major League Baseball.

The league grew from the ashes of the failed National Association. It got its start at a Feb. 2, 1876 meeting organized by Chicago owner W.A. Hulbert in which the NL’s original eight franchises agreed, among other things, to discourage the gambling that had been prominent at N.A. games, to “make base-ball playing respectable and honorable,” and to end the practice of forfeiting games by teams that had fallen out of the pennant race.

The N.L.’s first constitution was presented, ratified and signed at the meeting, and representatives from the eight franchises all left with copies of the agreement. But until recently, the original document existed out of the public eye as it was passed down through generations in the family of a longtime NL executive. Acquired by sports-memorabilia auctioneers SCP Auctions, the prized find will be sold in an online auction set to run from May 24-June 10.

“This is a huge revelation,” SCP Auctions vice president Dan Imler told USA TODAY Sports. “This is not a document that has made the rounds publicly. It’s never been exhibited in a major institution, it’s never been sold previously in a prior auction. It has never appeared anywhere. Obviously people are aware of various records of these documents, that went into print form, but there has never been any public knowledge of the existence of these documents in their original form.”

Authenticating the constitution necessitated a months-long process that included confirming the age of the ink and paper, handwriting analysis, and consultation with MLB historian John Thorn. Last year, SCP sold a set of 1857 documents representing the original rules of baseball for $3.26 million on auction. James Naismith’s original “Rules of Basket Ball” sold for $4.34 million in 2010.

“Very few documents of this importance exist in tangible form,” Imler said.

The early version of the National League faced competition from leagues like the American Association and the Players’ League, which only sounds like an Outkast album. In 1903 the NL began cooperating with the American League, its most successful rival organization, but the two leagues did not formally merge until 2000.