Nice car! On the gas cap though, its not the wrong one. My red 73 Mach came with the same cap. Not all Mach 1's had the pop open.
I honestly thought all Mach 1's came with flip open ones, sorry my bad!
The flip-open caps were only in '71. I believe there was even a recall where the '71 models which had them were supposed to be replaced with the standard cap that was used until '73. It was due to them popping open in an accident and spewing gas out of the filler neck. This is going from memory- I haven't Googled it to see if there were additional details.I honestly thought all Mach 1's came with flip open ones, sorry my bad!