I've got an original one still in the box! I believe it's aftermarket, as there is no FoMoCo name anywhere, or any company name for that matter. It is an old box and has the following on it:
No. 158 or No.159
MUSTANG
GAS DOOR GUARD
Installation Instructions
(1) Unlock and open the trunk lid. Take off Gas Tank Cap.
(2) Note that the Anti-Theft Gas Tank Cap is secured to the body of the auto by a bolt on a multi-strand wire. This bolt goes thru a hole in the lower right hand corner of the Filler Pipe Flange, and the nut for this bolt is on the inside of the luggage compartment. Unscrew this nut and detatch Gas Tank from the auto.
(3) Remove the four (4) self tapping hex head screws in the Filler Pipe Flange.
(4) Place Gas Door Guard over the filler Pipe Flange so that the section of the outer circle on the Gas Door Guard which is flattened and beveled goes to the top and is parallel to the line of the trunk. Make sure that the embosses in the Gas Door Guard go over the embosses in the Filler Pipe Flange, and that the holes line up.
(5) Replace the four (4) hex head self-tapping screws. Replace the Gas Tank Cap by inserting the bolt which is attached to the multi-strand wire thru the lower right hand hole, and secure with the nut on the inside of the luggage compartment.
I can e-mail a picture of it, if you would like.
Rich
1966 GT convertible