How to add female 39mm threads to the CV 15 to take a 39mm IR filter:
1. Remove glass from any B+W 39mm filter (B+W filters are brass, which the threads don't bind with another filter as easily as aluminum).
2. Grind down the male
threads flush with the landing.
3. Use a little
5-minute epoxy to glue it to the front ring of the lens, just inside the fins (hood petals?).
4. UV/IR filter can now be attached in the normal manner, won't fall off, and will come off easily whenever you want, like any normal lens, and the adaptor will not vignette on film. If you use a Leica-branded IR filter, don't screw it really tight because Leica filters are aluminum and they do tend to bind on.