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I have a focal plane shutter Topcon uni with what is probably the same lens. Mine comes off easily. I don't have to push, rotate, release, and rotate; I simply push, rotate, and off it comes. It rotates less than 45 degrees between locked and removal orientations. It's very much like a Kodak Retina DKL mount, which can have problems if the aperture control tab thing is out of place or out of shape. I had a Retina that would let the aperture ring rotate all the way around. It was a pain to get it working right. OTOH, my Topcon is built like a precision tank, and it weighs like a tank, too. Try futzing around with the aperture setting, while you try to take the lens off.