I've just had a quick go at experimenting to see whether the filter would give an accurate reading when using a Voigtländer VC II meter. I tried a reading with the filter over the lens of my M3 and got a reading of 1/30th at f2.4. Putting the filter into the view of the meter gave a reading of 1/8th at f2.4. I then tried the same experiment with an M6. I got the same result with the filter away from the lens, but when it was placed over the lens, I got a reading of 1/15th at f2. I believe that the frame of the filter is interfering with the VC II reading leading to over exposure. It would have been nice if it had worked!
Nick