Just for information for people, Copal made two variants on the Copal 3 shutter; 3 and 3S. The 3 had 58mm threads for the front and back cells, and the 3S had 56mm threads. There's other specifications about them that are different as well (distance to iris, etc.)
However, I have seen shutters that were labelled "3" and some of them were actually the 3S specification, so apparently, they didn't follow the naming for shutters that were sent to a lens manufacturer for use in one of their lenses... so be careful if you are buying one to match to specific cells, because they may be different, even if they don't have a different label.
Good luck with the sale Dirk...