Letter High (1910)

“Not knowing what the youngster likes, the star twirler may feed him a fast one, letter high, only to see the bush leaguer jar the left field fence.” Billy Evans, Indianapolis Star, February 20, 1910, p50. (Syndicated story; also appeared elsewhere.)

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1ST USE. 1927 (Hal Totten, WMAQ-Chicago broadcast, June 17; Edward J. Nichols).