Size #3S shutters
I don't know why, but Copal made a bit smaller than #3 shutter, and named is #3S, where that 'S' stands for 'Small', I believe.
The #3 and #3S shutters seem to be identical inside, only connecting threads are different. But you should pay attention to this since these minor differences make them incompatible, Also, it is worth to measure your particular shutter threads since I had a few reports when #3 shutter had smaller threads (like #3S). Source of these issues remains unknown.
Their main dimensions are: