Originally Posted by Alberti
Or is it that there is a secret difference between Nikon-S and Nikon-S2?
No, all Nikon rangefinder cameras have the same mount - mechanically the same as Contax RF, but spaced and geared for Leitz (51.6mm) rather than Zeiss Ikon (52.3mm) normals.
By the way, the proper terminology would be Nikon RF, not Nikon-S or Nikon-S2. The latter is too ambiguous, as "Nikkor-S" is a nomenclature actively used by Nikon themselves for lenses, while Nikon have never used a dash in its rangefinder camera naming.
When searching eBay for "Nikon-S" you'll find far more seven element SLR Nikkors than Nikon rangefinder lenses. Up to the mid seventies, Nikon had a lens nomenclature built around "Nikkor-[first letter of latin word for the number of elements]", and most sellers don't make enough of a distinction between Nikkor and Nikon.