The 64 Mustang Body Assembly Manual shows a retainer plate, C4ZB-6510198-B, using 2 each screws but does not reference a screw part number. The 65 Mustang Body Assembly Manual shows a different retainer, C4ZB-6510182-B, using 2 ea (10 places) of rivet 378527-S, 1/8NOM,0.337BODY L,0.250HD,0.126-0.187GRIP,AL (66 uses a stainless steer rivet). The 75 MPC references rivets only.
This "may" be a 64 running change from screws to rivets.
Jim