I posted a question about the ring on RFF before reading your question here. Someone replied that it acts as a lens hood for the rear element. Taking it off might cause light bouncing off the film and could cause problems. We'll see after I get my film back--I shot with the 21 Biogon on my Leica IIIc with the ring off. I could have left it on, but it did rub on something inside the IIIc when I first mounted it.

Bill