I think Larry answered this one but looking at that picture I see something chromed or shinny lurking in the right rear window location?
If that is what you are talking about then it probably isn't the fuel tank filler but the top of the overflow tank for the cooling system, BUT, the factory race cars of the era, referred to now as the Gp4 cars had two fuel tanks in them.
I never saw details on exactly how the factory modified the fuel system but it sounds like the original fuel tank was left in place, an external filler neck with a Cobra type quick release fuel filler was added, with a corresponding one on the right (passenger) side.
I do not recall ever having seen an original set up in person, pictures of one, or even the parts book illustration of the additional parts.
In any case, if this yellow car has a fuel tank added on the right side it has a high probability (99.9%) that it was added by a previous private owner, BUT where that "cap" is in this picture, is where the additional "fuel tank" would be located.
Hope this doesn't confuse the issue, the matter, or the previous responses?