What some people don't realize is that all the Leica lenses with Goggles will work on all "M" Leicas WITH THE GOGGLES. (...well except on the CL and possibly M5)
Just because you have an M2, or such, doesn't mean you need to source a lens made for use without goggles, as, for example, the 35mm Summaron with goggles will work and function just fine. It brings up the 50mm frame, and then the goggles turns this view into 35mm field of view.
This bit of "insider" information could mean that you can find a bargain out there somewhere.