As Ko.Fe noted, the 35mm Skopar comes in three variations, two LTM, one bayonet, optical design is the same for all lenses. The first LTM version, sometimes referred to as "pancake", has the 43mm filter thread, the second LTM version and the bayonet version have a 39mm filter thread, which is standard for classic Leitz M lenses. Between the two LTM versions, the second one is preferrable because of the closer focus limit and is usually a bit more expensive on the second hand market. It is the one I own now. I used to own the M bayonet version, the lens developed a problem, it literally fell apart as some internal screws became loose. I also had problems with a certain type of flare with that lens with the sun at an oblique angle outside the frame. My suspicion is that it was caused by the shiny chrome lens hood mount. No such problems with the LTM lens. Optical quality is quite good. I use it only rarely as I normally prefer the Canon 35mm f2 which I find is even better value for the money than the Color-Skopar.
