The meter in the M6 reads the reflectivity off of the white dot on the shutter curtain. The depth of the rear element of the 3.4 Super-Angulon has two effects with the M6. First, it places the rear of the lens much closer than other lenses, which is why it couldn't be used with the M5 or CL. Secondly, the rear barrel blocks most of the chamber so the meter sensor may not get an accurate reading. You are correct that the digital Leica reads the light from a sensor directly behind the lens. It's a good example of what the lens is capable of. If the meter in the M6 is incorrect with the SA, you can always use an accessory meter.