I am starting to get confused by the variety of Zeiss Contax rangefinder Biogon models after thinking I had it all understood. I know that the pre-war Biogon and the post-war Carl Zeiss Jena Biogon do not fit the Contax IIa and IIIa. However, it is my understanding that on at least some post-war Jena 3.5cm Biogon models, the large convex rear element is protected by a guard ring, rather than being completely exposed as in earlier models (and on the Jupiter-12). Can anyone here verify this? Also I thought this rear element came right up to the film surface, almost touching it.