The movable patch of the rangefinder on screw mount Leicas & on most M3's only travels from Infinity to 1 meter.
To have a 35mm lens focus closer on an M3, what the engineers at Leitz did was: Build a mechanism that would physically move the lens from where it was focused on an image plane at Infinity to where it was focused on an image plane at 0.65 meters while the cam in the lens moved the rangefinder patch in the camera reflective of a distance from Infinity to 1 meter.
This lens was attached to a set of "goggles" which fit over both the rangefinder window & the viewfinder window. These "goggles" expanded the angle of coverage of the viewfinder by a factor of 1.5X. This meant that everything seen thru the "goggled" viewfinder was now seen as being 1.5 times wider & 1.5 times further away. Things also appear to be smaller.
The 50mm frame lines seen in the viewfinder now showed the field of coverage of a 35mm lens.
Because of the minified image coupled with an unaffected rangefinder & unaffected frame lines: The subjects being viewed were now seen by the rangefinder as being 1.5 times as far away as they appeared to be without the goggles.
Therefore when the rangefinder mechanism said it was measuring 1 meter it was actually measuring a distance of approximately 0.65 meters.
The lens mount is engraved accordingly.
Note: 1 divided by 1.5 is 0.666666+. This is the reduction of viewfinder magnification by the "goggles" to the original system.
Note: The 0.91X viewfinder image magnification of an M3 multiplied by 0.666666+ = approximately 0.60X. This is the viewfinder image magnification in the range/viewfinder window of an M3 with "goggles".
Edited by Michael Geschlecht, 21 March 2017 - 18:13.