Well after a lot of dithering, I decided it was time to go ahead and strip that strange rusty metal primer off the rear right fender to see what was underneath. I also found some some of it along the car corners.
Things did not come exactly as I would've liked. I would up with a strange residue on one cap corner that just looks weird. And there was so much blue/black primer on the fender that I could not get through it even with about 10 applications of paint stripper.
In retrospect, this was a good lesson in "Stop when you're ahead!"