I have another question about the 90 Hexanon (which, BTW, is gone to the repair-person with my M4 to be calibrated): I'd like to use 43mm filters on it using a "step-down" ring, so that I only need one set of 43 mm filters (my two other lenses have 43mm thread)

If you have tried it, I'd be very happy to get a confirmation about the feasibility... When I put a 43 mm filter on the lens, the filter does not hide the front at all, so I guess it should work.