Witch gas cap?

Top one, depending on what the design is on the backside. Filler cap/tube design varies depending on year, as well as some possible variations dependent on destination/location emissions requirements.

Second one is aftermarket type which may functionally work, but not factory piece, and again depending on what design is on the backside.

If the back design is wrong, it will typically not mount, fit loosely, or not lock properly.

Post some pics of the backside, and we can further confirm.

Do both mount to your 72, and is the fit easy yet snug?