research: http://www.straightdope.com/mailbag/mfuckina.html says 1947 is 1st recorded print version...spelled "ay" by Norman Mailer.

I heard it in 1970 as "aye" and knew immediately what it meant. Obviously, some form was used, as the article says, in WWII or before. I, frankly, am not willing to believe that it originated as "A", and later transformed to "aye" by a few.

The exact answer may never be known, but unless one IS found, I will continue to claim that 'aye' makes sense and is a more likely original.