In desparate need of early model 71 dart fuel cap

Do early '71 Dusters have the same problem? I have and early model '71 Duster that I picked up and the fuel cap doesn't fit tight and wobbles. I have tried caps from my other non-'71 cars and those caps wobble too. By looking at the filler neck it looks like the "ramp" for the cap "tangs" don't ramp up as high on the '71 as it does on my '70 and '72.

Chuck