What's the history behind these caps? Not aircraft far as I know. Seems like they may be stuff for an Indy car of the 30's or 40's.
No specific history but there was a fully restored early 30s Deusenberg factory race car at one of the Jalopyrama's that had one very similar to your picture. I can't imagine anyone using anything that was not correct on such a valuable car. My point is that they go waaaaaaay back. I did find a picture. You can see it behind the drivers seat silhouetted against the wall.
I was looking at a Moal unit awhile back....top notch workmanship....google it and check out some of their rides they've fabbed up...there was a garage crawl that visited the moal garage that was documented on the HAMB awhile back as well....great stuff... I digress...while not aircraft persay they were produced to mesh up with the industrial era as well as the old school racing era....me? I like it!