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Thanks Michael. Yes, I noticed focus shift with the 75. I think Cosina optimized it at around f/2.8 for rangefinder focusing. If optimized for f/1.8, then there would be noticeable back focus as it's stopped down until depth of field is deep enough (not sure when it would reach that point). I've only tried this one copy, so I wasn't sure about this, but it's interesting to note that you've had very nearly the exact same experience as I have. I did a test where I photographed a sequence with the camera on a tripod from wide open to stopped down, and the focus shift was pretty noticeable.
A few months ago I found a reasonably priced 90 Summarit and now use it more than the Voigtlander, which I will likely sell. If you're looking for modern lens performance, such as good across the frame sharpness, then I think the Summarits are a better option. The 75 Summarit is supposed to be even better than the 90 and both are smaller and lighter than the 75 f/1.8, which I honestly think is an f/2 lens because I can see no difference in exposures between f/1.8 and f/2. With some M9 experience under my belt now, I also believe that the jump from 50mm to 90mm is more comfortable than the smaller change from 50mm to 75mm.