“Dunder, keystoner for New Orleans, is being touted as the best second-sacker in the league.” B.H. Mooney, Birmingham News, May 4, 1908, p7
Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1917. “[Nap Lajoie is] the king of the keystoners” (The Sporting Life, March 10; Peter Morris).