I think I read (likely here), that the cal. 321 was last manufactured around 1960, though they were placed in cases through approx. 1968.

So I'm chatting by phone (I know, pretty retro, huh?) with an old-time clock restorer who swears that he has a bulletin somewhere that says that around 1968, therefore at the end of its run, Omega was still making upgrades to the movement, including, according to him, going from a Breguet hairspring to the flat variety. Any truth to this?