[Leica] Lens/Filter questions

[pklein at 2alpha.net (Peter Klein)]

LUG:  A couple of obscure lens and filter questions:

1.  One of my favorite lenses is my 50/2 Dual-Range Summicron.  Which will 
not mount on a certain digital camera.  What are the chances of my finding 
a broken or bad-glass Rigid Summicron, into which I could screw my DR's 
lens head?  How much might that cost me?  (I realize the mount focal 
length would have to match).

2.  Is there such a thing as a 40-39mm filter step-down ring? That's 
forty, not forty point five.  The purpose would be to use a 39mm filter on 
an old Canon 50/1.8 lens. (If not, I might have to resort to the "dental 
floss in the filter threads" trick, but it's a rickety solution.)

3. I have the early 1980s 90/2 Summicron with a 49mm filter thread.  If I 
were to use a 49-46mm step-down ring (which I know exists) and mount a 
46mm filter on it, would it vignette?

Yes, these questions involve that certain digital camera, adapting lenses 
to same, and trying not to buy to many off-brand filters while waiting to 
get the free ones.

--Peter