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I've only seen pop rivets. The one time I saw screws was on a GT convertible that someone added rocker panel moldings.
Which makes sense in that the clip kit in the period parts and accessories manual calls out 20 8-18 x 1/2" pan head screws, 376389-S101(UU82D).
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
A point of reference: the AMK "Guide to Ford Fasteners - 1955-73" and the "Ford Standard and Utility Parts Catalog, 1969" both have the length of screw 376389-S101 at 3/4 inch.Not unusual, but Ford does make "errors".Jim