I'm looking right now in my 1972 M5 instruction book, in front of me. "The next smaller focal length frame functions as the metering area". "For a 35mm lens, the light-metering field is indicated by the 135mm frameline".
When I had my 21mm CV lens, and used it very successfully on M5, I used the central block to take my meter readings. But, it is very spotty, IMO, since the field is -so wide-. But, I was always happy with the results, due no doubt, to the -awesome- M5 spot-meter accuracy. Absolutely uncanny.