Where the plate bolts to the AFM - if there are 3 8x1.25 holes in a triangular fashion that holds the plate in place - that's an early - I think 77 and 78 AFM. If it has 4 holes on the bottom of the AFM in a rectangular fashion, then it's a later model - 79+. Someone will probably correct me, but I just replaced mine and they have an adapter plate now that sandwiches in between that is drilled for the 3 hole to mount to an early style Z.