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Yours look like mine. I use this type cap that I sent ya earlier. Show pic of your gas caps. Go to the Stant website. They show part #10806 is for Apr or older 1971, May or newer is part#10807. Do you have both 1971 gas caps? I read an article about the differences in filler necks and I don't remember where I found it. The 71+ yrs, the fuel cap area protrudes farther out. From what I see, yours look like a 70. For 1970 the part# is 10623 w/o Ca emissions. http://www.carid.com/1970-plymouth-barracuda-fuel-systems/item-2104916.html?
If this helps, I measured my fuel tank inlet. It protrudes out appx 3/8", outside diameter 2 3/16", inside diameter 1 3/4", and the inside lug to lug 1 7/8". This is for a 70.