All, The gods smiled upon me and I was able to purchase a #5 Betax shutter. It comes with the retaining ring (Actually the ring that screws onto the lens board via small machine screws). Does anyone know the ID dimensions of this retaining ring ? Im again assuming that all #5 betax shutters were the same in this dimension. Basically I just want to know how large of a hole I need to cut into my lensboard. Thanks !