So under my hood on the left hand side I found one plastic red cap the covered the screw threads sticking through from the inner fender.
A year or two ago I found a guy who had found a NOS package of those caps that he was not sure where they went. So between us we confirmed these are real and should be covering the threads sticking up from inside the engine bay.
Well I only had one and I accidentely covered it in epoxy primer before I knew what I was doing.
But I found these and may order a set to see if they will fit. They look close. Just a little more texture then the original.
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I do have a set of these nos red plastic caps for the threaded ends of the screws you mention. Bought it many years ago on Ebay.