"And it is definitely not true that the later ones are 2 stops more sensitive. That change occured with the M6TTL."

You got me on this one: It's a one stop difference not two stops. This information is from Leica-Literature picked up in a local store in 1988 and 1996 about the then current M6 and there is a 1-stop difference between the numbers given (EV 0-20 vs. -1-20).

This is also confirmed in an older thread in the LUG-archives. Well, maybe coincidence... http://leica-users.org/leica-users/v26/msg05808.html

If the circuits in your Wetzlar-M6 have been updated the sensitivity of both meters, Wetzlar or not, will be identical of course.