I have heard of them getting the digits reversed in 39 like "93" or whatever, but I can't prove it happened. I have never seen or heard of one reversed in quite this way.
I will say this, it looks identical to my 1939 and 1941 marked Schwinn cranks. The 41 has AS&CO on the rectangular boss on the back side, and the 1939 is left blank back there. I believe both have the F mark in the square. I know the 39 does because I just looked at it. Neither crank has any reversed numbers.
I am no authority on these things, not even close, but it does not seem an impossible stretch for a crank forged in 1939 to wind up on a 1940 bike. YMMV.