I have an old 1920s Dial Set shutter that is clearly Size 0 in all dimensions, lens 29,5 front and backside,
and I have two Rim Set Shutters from the 1930s that have equal outer dimensions to a regular 0, and the front lens mount is also 29.5, but the rear lens is smaller (but larger than the 22.5 of the size 00)
Also the back mount size of the shutter is smaller than the 35.5 of 0, but does not fit into a 00 plate, the size is about 29mm

Have not seen this size listed anywhere yet?