The F-2a is the same design as the F-1a. The reason for a different designation is the more rounded lower corners of the flap. The thought is that the order was placed with a different vendor and the difference came from the difference in the vendor's production of the flaps. There is a controversy about the existance of a F-2b. If there a F-2b, the only difference is the F-2b is a shade or two lighter. The jury is still out on this fact.