I like the theory and I want it to be true because it's funny and creative, however I am not convinced. There are too many modifications to the molds, they would just end up making new molds. The real killer is the holder for the marker light bulb. You even indicated that it doesn't seem to fit the theory.
I would bet that the ring feature isn't to seal it to a distributor housing but more to seal against a different lens shroud and the missing tab isn't to fit the cap to a distributor but actually to help align the light at the factory. I don't know either obviously but my point is that I can come up with other reasons that could also be possible that its not a cap.
But yes I am hoping it is a cap because that is hilarious.