Very interesting, Wimpler. I'd measure those mask flaps -- they're more likely to be for 6x4.5 than 6x3, and your camera probably has had the back switched. The Isolette model that had the 6x4.5 masks and VF mask had two windows, one in the center at one at the bottom (for the 6x4.5 track that was on all all 120 film by 1953 or so). These were all Bakelite top plate cameras, AFAIK...