If you want to us a 40mm the early M frame for 35 mm is so small that it is serviceable for 40mm.
Originally Posted by Renato Tonelli
The M6 frame is closer to 35mm think the change was between M5 and M6.
To use this you need to nail file the lens mount as it comes out of the box as a 5cm lens for frame selection.
And the M needs to not have been retrofitted with a late finder.
I used an elastic band to hold the selector lever over, between the strap lug and lever had several in gbag, I preferred to keep the lens original but lots of people buy a CL kit and sell the body