We found the cap today and it is an exact replacement. Autozone didn't have anything like it but for once the directions to a place that would have it was spot on and right across the street !
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(Thanks to Google Earth
for the picture)
It's become a joke with us that whenever a store doesn't have something they don't want to be rude and leave you hanging so they always suggest who will have it, even if is very unlikely. Likewise if you ask for directions you'll get helpful suggestions even if they have no idea where it is. It's a different kind of politeness and it's put quite a lot of wear on our shoes but we're not complaining, it gives us lots of exercise.
The new cap is as I said an identical replacement, including having a metal to metal seal and no gasket at the filler throat. The metal in the cap looks and feels like thin soft bronze or brass.
I'll make sure to really clean off the mating surface and will cross my fingers.
Thanks again for all the comments and suggestions.