Is this a "real" pop open gas cap?

This was on the car when Dad bought it in 1992. IIRC, from my childhood not paying close enough attention to Dad memories, he said that the guy who built the car said it was NOS. What say the experts?

Sure looks original! From my understanding the originals did not have the internal screw on cap. I would say that since those "NOS" products became available in the early 90's, and the condition of yours, it is original, or at least an original cap that may have been swapped early on. Are you going to keep it on there?

That's definitely an original or nos one, the caps were offered in 2 versions, the 71 through early 72 or so had the rubber gasket part of the flip down door like the one you have there, sometime in mid 72 until 73 the caps were made with a small screw on cap underneath like the reproductions have. The change was made because the supposedly the early style had a tendency for the fuel to slosh out past the rubber gasket since it wasn't a tight seal, to fix that the made a separate screw on cap underneath, at least that's what I've heard accounted for the change, with that change the part number also differed the early one like you have carries a D1ZB part number while the later one and all nos ones I've seen have D2ZB.

I am disassembling a 1972 Mach 1 and just removed the pop open gas cap last weekend. My cap was rusted as well but original to my knowledge. It looks exactly like yours pictured. I believe you have an original or NOS. Either way - you likely have the real McCoy. My opinion is that the pop open caps look nicer than the twist off.

That looks like an original

Look at the P/N on the back if you take it off

The pop open caps only came from the factory on 1971 cars.

Oh, I'm swapping it for a small cap repo. It has popped open after when driving after filling it before, and, if I fill the tank, it will leak fuel under hard acceleration.

This is the thread that sparked my question: http://www.7173mustangs.com/thread-gas-cap--16305

If your swapping your cap to a repro style what are you planning to do with the old one? Been looking for a cap for my 72.

Stash it for now.