I'm not the expert, I do know there were changes. Look at these two old thread, there is lots of info there. I have a 74, but the cap (which is bad) is not good for an example, because it's an aftermarket locking cap. Fits real sloppy, gasket is bad, and it might not even be correct
First, I'm adding 3 more photos at different angles to show the tangs on the cap. I don't want to mislead anyone to try and sell a part. You can see two tangs are curled over. There are many threads on this issue, so I'll go ahead and add my 2 cents worth. It only took me 10 years to restore my 71 Duster340. During the restoration I found many parts were interchanged and not correct, including the filler neck. Anyway, what I know for sure, this cap shown here, does not fit my Duster that was mfg. 9/70...
Up to early 71 the vapor separator was used and a non-vented cap for the filler neck. My car now has the vapor separator. My understanding was somewhere around May 1971 the Vapor Saver System, which includes the vapor separator, was changed to the 72-76 evaporation control system and the filler neck changed at that time, thus changing the cap. So therefore 72-76 cap would be the same.