Some of the prototype vehicles made by the manufacturers have trickled out over the years, and some of them were pretty amazing.
I knew one guy who got his hands on what looked like an ordinary 4-door sedan of the early 1960s. What made it special was that it was solid steel, with non-standard reinforcing that made it into a real tank. Oddly enough, its fuel mileage was not very much worse than a production model of the same type. The difference being that the engine was hand crafted, not mass produced, so had no tiny gaps by which a lot of efficiency is lost.
He was not a good driver, though, and got into four accidents in the time he owned the car, while seriously damaging the other cars involved, just scratched his paint.
This isn’t quite the same of that, but a friend of mine owned a ‘64 Fairlane back in the day that had finned drums, a 289 K-code engine, and a modified C4 automatic. It was an adventure every time he needed parts for it, especially for the transmission. After blowing up the tranny using standard C4 parts in it, he converted the car to a 4-speed.
He found out some time after he got rid of it that the car was a homologation special for European stage rally.