Nate, Leica M-Mount is 39mm, somewhat smaller than your typical modern bayonet mount. It does look huge for a Leica lens, though. Almost certainly blocks the finder a bit.
"This myth that its immoral to use one on a m-mount camera has gone too far."
There are MANY myths about M-Mount cameras, like, "Owning a Leica makes you an excellent photographer." There needs to be someone to give a rich person a whack on the wrist every time they buy a Leica for show. Bad rich person! You're driving up the market value for the rest of us!