Leo Mangum 1924 - I found the image
here, and quoting from that page:
"This 1928 photograph displays, then, Giants pitcher Leo Mangum. Astonishingly, Mangum pitched in only one game as a Giant - allowing five runs in three innings of work. So not only is this impressive image a shot taken by the legendary Charles M. Conlon, it may be the only picture of Mangum in a Giants uniform currently in existence."
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edited to add: I have been advised by
tnfoto that while this is, in fact, Leo Mangum, it is not a New York Giants uniform... rather it's a White Sox uniform with the breast logo obscured.