excluding multi-player / team postcards and cabinets, here's a few individual player postcards/cards that pre-date their established "rookie cards":
1909 RPPC in Superior Blues Uniform - Dave Bancroft:
1967 Cincinnati Reds Team Issue PC - Johnny Bench:
1955 JD McCarthy PC (postally used in 1955) - Jim Bunning:
c1943 RPPC in Monterrey Uniform - Roy Campanella:
1914 University of Michigan Photo Card - George Sisler:
1950 Coppedge Studio Winston-Salem Cardinals Team Issue - Earl Weaver:
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http://imageevent.com/derekgranger
HOF "Earliest" Collection (Ideal - Indiv): 244/342 (71.4%)
1914 T330-2 Piedmont Art Stamps......: 114/119 (95.8%)
1923 V100 Willard's Chocolate............: 177/180 (98.3%)