A Colt Service Ace upper end that came with the first handgun bought on my own, a mint GI 1911A1. I paid $200 for it, the conversion unit, and a bunch of GI ball .45 from a schoolmate's Dad. Being young and dumb, I decided I didn't need a .22, and I listed the conversion unit in the paper for $50.
Sold it at 6:30 AM the day the ad came out. Got calls on it for months......
What a dummy....