The organization became an entity with (Robert) Davids’ March 19, 1971, letter:
This letter is being addressed to about 25-30 persons interested in baseball history and statistical research (I use the term “statistorians”). You are an addressee because I have seen your name in The Sporting News in past years, appended to an interesting historical or statistical article, or your name has been passed on to me by Ray Nemec, Bob McConnell, Leonard Gettleson, or Cliff Kachline.
There may be many more than 25 or 30 baseball statistorians around the country. We don’t really know, but I thought some effort should be made to organize this “motley crew” into a more formal group. For that reason we plan to hold an organizational meeting at Cooperstown, New York on August 10-11, 1971. Cliff Kachline, Hall of Fame Historian, has kindly invited us to meet in the museum library. The Hall of Fame baseball game and induction ceremonies will be held on Monday, August 9.
You can read more on the history of the Society for American Baseball Research here.