All I can say is LewisWatchCo used to buy these bezels from Omega to fit them on a variety of mods/restorations for clients. That is, until Omega made them exchange only.
Outside of that, I bought a new 1861 directly from Omega last year and it came with the 3861 Don bezel on it. It did not have a gasket. It also fit my ‘84 861 Speedy but I didn’t take a picture of that.