Left Fielder (1862)

“L.F., left fielder” (in list of abbreviations following a pair of box scores). “Base Ball,” Rocky Mountain News (Denver), April 29, 1862, p3
— The list of abbreviations is preceded by this text: “For the benefit of those are not posted on the contractions pertaining to a description of the games, we give them as follows:”

1862 box score abbreviations in Rocky Mountain News

Previous earliest use (Dickson Baseball Dictionary, 3rd edition, 2009):
1864.  “The good looking left fielder of the Eurekas will have to drop the ornamental style and adopt the useful in fly catching if he desires to keep up his past reputation.” (New York Clipper, Aug. 13; Peter Morris).