I searched for a filter setup to use in the filter threads of my II and IIa, and found the Tiffen Series V adapter. This takes all the Series V filters of course and mounts on both the Schneider and Rodenstock lenses. Diameter of the threaded end of the Tiffen adapter is about 30mm.
I have the press-fit series VI adapter with filters and hood as well, and they work on the IIa, but not on the II (a little loose fitting).