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JacksonThere's a great resource created by Marshall Fogel, Henry Yee and others which describes the history of sports photography, including, in great detail, the differences between wire and news service photos, as well as Type 1, 2, 3, etc. The book includes scans of varying stamps which you may find on the reverse of either photo type(s) among other things.
Mastronet carries the book. Mine is beaten up pretty good. I refer to it often.
HIGHLY recommended vintage/original photo collectors.