kuuan
loves old lenses
CV 4/21 and 4/25 do great on the GXR M too. Like the 15mm my 25mm has M39 while my 21mm Leica M mount. I prefer the handling of the 21mm, that is the M version.
Advantages of the M39 versions are smaller size and that they can be used on M39 bodies.

inside Lang Suan railway station by Andreas, on Flickr, CV Snapshot Skopar 4/25
Advantages of the M39 versions are smaller size and that they can be used on M39 bodies.

inside Lang Suan railway station by Andreas, on Flickr, CV Snapshot Skopar 4/25