I noticed that too, that frame looks like the one used when they went BMX, except for the fork, the BMX bikes use a longer one, like a 24" fork. I'm assuming this bike is all original except for the paint scheme. Also, the dropouts look like later versions. The BMXers also used a plastic chain guard, yours is metal. AMF attached their sissy bar to the rear holes of the dropouts, yours has it attached to the rear axle, maybe they put it back together that way. Your kickstand looks like a 60's model.
It's a mystery, the serial number is the best way to ID it. Look right here: