The clip is shown in the "Ford Standard and Utility Parts Catalog, January 65" pg 35 (also the 66 Catalog, pg 35) as p/n 378523-S32 with some basic dimensions. It is an alternate part in the 65 and 66 Mustang Electrical Assembly Manuals (pg 22, pg 27) for the screw attached clips. There is no heater page in the 64 Manual.
Jim