Thanks to all for valuable information on the topic. Some years ago, I sent my M3 to Solms to get the rangefinder images properly aligned, and got it back, not only with the alignment in order, but with a new viewfinder with frames for 35, 50, 90 and 135mm lenses, plus a new shutter as well (Needless to say that there also was an unexpected invoice...). The camera works like new, but the adding of the 35mm framelines without changing the original M3 magnification of, I believe, 0.91, forces me to move the eye around to control what gets into the picture when I use the 35mm. This problem, with the inconvenience of having to use a handheld meter, makes me consider acquiering a MP. I believe the 35 frames of the 0,85 should be more easy to see wearing glasses than those of my renewed M3.
However, the great and clear finder image is a dream with the 50mm and the two longer lenses.