The Nikon S-mount was used for it's line of rangefinder cameras - almost all of which from the early-1950's until production ceased were nomenclatured as "S" models (e.g. S2, S3, SP and S4).
Originally Posted by Bill Mitchell
The S-mount is compatible with the Contax C-mount up to the 50mm. Beyond that they are incompatible because of slightly different film planes. Contax cameras were made by Zeiss.
If you look up the recent thread started by Snegron regarding his ill-fated attempt to purchase a S2 you'll find my post which went into detail about the S-mount and C-mount etc.