Baseball Cap (1865)

“Base ball caps!” (Nicely’s Emporium of Fashion advertisement). Baltimore Sun, September 28, 1865, p1

Base ball caps in 1865 advertisement

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1889. “A base-ball cap free to every purchaser” (M.J. Flavin & Co., San Francisco, advertisement for “outing suits.” Daily Evening Bulletin [San Francisco], June 10, 1889).