I recently bought a Spiratone Expandar 28mm f/3.5 lens that was marketed as an M42 lens. Sure enough, it mounts and focuses properly on M42 cameras. I noticed, however, that it appeared to have an adapter on it so I concluded that it was actually a T-mount lens with an M-42 mount. So I unscrewed the adapter and found that the lens mounted just fine on a T-mount for my Canon EOS (that is, it had a 42mm by 0.75 pitch on the rear of the lens itself), while the camera to mount thread was, as stated, an M42 (42mm x 1mm) However, when I tried to focus on a Canon body with a T-mount on the lens, the lens would focus to infinity only at its minimum distance, and would not focus any closer than infinity. I noted that the lens mount was actually marked CS, and I have another Spiratone wide angle (35mm) with the same adapter. When I measured the "CS" M42 adapters, the distance from the rear of the lens to the front of the body is in fact something like 9.53mm instead of the T-mount distance of 9.37mm. The M42 screws of the CS mounts (at total=13.65mm) are also taller than on the true T-mounts (12.38mm). I've googled both this site and generally, but all I can find that is even close is the much smaller movie/astronomy lens mounts known as C and CS which have considerably different diameters and screwthreads. I presume the Spiratone CS is a proprietary mount that was special for the Spiratone preset lenses, an in-house variant of the more common T-mount. I do have some later Spiratone lenses (such as the 200mm) that are true T-mount lenses. Can anyone help me out here, and tell me if I've got it right or wrong?