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Dear Mr. Garner,
Thank you for posting your request on History Hub!
It is difficult to determine what S.O. means without knowing the information captured on the negative (besides the numbers provided). Usually, S.O. means Special Order in Army records. Is it possible to make a photocopy/Xerox of the negative and send it to us?
It could stand for "Signal Officer" (Army Signal Corps), but we need to know the content of the photo in order to establish a context.