Attached are copies from the 1966 Electrical Assembly Manual, and individual Parts and Accessories Catalogs from 1964, 1965, 1967 (I don' have access to my 1966 at the moment), and the 1960-68, and 1965-72 MPC's.
Note that the Assembly Manual only shows this on the Passenger side, and the 1965 Parts and Accessory Catalog confirms this.
Note that this was used on many Models (not just Mustangs), and was only used on 1964-1/2 to 1966 Mustangs (and apparently, not just Convertibles) ; there is no listing for this pad in the 1967 Parts and Accessories Catalog, nor does it show up in the 1967 Electrical Assembly Manual. The other pad/ gasket listed (13A790-A) did not replace the 13A739-A pad, but was for a different location ; I believe for the door mounted courtesy lights for the Deluxe Interiors of 1967 and 1968.
Interesting find though. I would bet that not a lot of them have survived.
Bob