Typically barrel engravings relate to the film-plane. As the M3 has no film plane indicator nor does the manual hint at this issue you might place a costomized groundglass/foil screen on the film rails and check for yourself.
Measure to the lens mount then add 27.8mm assuming you are checking focus. The M3 can be modified easily to focus down to .7m. I did it to mine but I don't remember exactly what to do off the top of my head. I think it involved bending the stop for the rangefinder cam arm. I would really recommend it.
From the film plane, same as any and every other camera.
Yes. I wasn't clear (stealing posting time at work). I was wondering if there was a film plane indicator mark or 'tradition' on the M3. Like the struck-through O on lots of cameras, or the top of the serial number on the Nikon F.
I photograph things to see what things look like photographed.
- Garry Winogrand
I had the same question and called at the "Leica Customer Service" in Solms. They could not resp. did't want to answer my question because this is considered as a company secret. They recommended me to phone Carl Zeiss in Oberkochen. They were very friendly and confirmed me that it is common practice to measure the distance from the film plane.