I just received a beautiful-condition 1950's CZ 21mm Biogon and intended to put it on my Black Dial Contax IIa. I set the shutter to T so I could see just how far the lens went into the camera, and found that the little ring with ears at the rear of the lens would hit the top and bottom of the lens chamber when focussing the lens. Then I found that in fact the rear ring-with-ears rotates. Can this be correct? Can Zeiss have manufactured this lens with this ring and ears knowing that it would hit the innards of the lens chamber, then gave the ring the ability to rotate? So that at some point the ears hit the top and bottom of the lens chamber, and the lens can keep turning/focusing because the ring rotates around the rear element? I can avoid this by putting the lens on my Nikon S2, which reportedly has larger lens chamber than does the Contax IIa. But I'd like to try the Contax, IF this is really how this is supposed to work. Thanks, Contax experts!