"Clip on" retractors were shown for factory assembled convertibles only, C6ZB-76610A88, and appear to be for the deluxe belt option only (Ref Osborn 1966 Mustang Interior Assembly manual, page 37). Convertibles had their own seat belt part numbers , Fastbacks and Hardtops used the same belts.
A previous discussion determined that 65-66 deluxe belts also included the seat belt warning light.
An interesting note on this drawing, the FOMOCO label was to be on the underside of all belts.
Jim