The SP and the S2 also have a 1:1 finder, the M and S do not have this feature.

I dont find this feature to be too useful, as I always shoot with one eye closed. Some of the parts were used in the Nikon 'F' SLR that hit the market in 1959, by 1974 they had made 1,051,048. Nikon started with the 'I' in 1948, 'M' in 1949 & 1950 and they made 2 model 'L' in 1950. The Nikon S3 has a 1:1 finder, so you can shoot and compose with both eyes open. Nikon made 36,750 'S' rangefinder cameras from 1951 to 1955, then came the S2, Sp, S3, S4 and S3M.

The SP also had a second viewfinder for wider lenses next to the main viewfinder. The S3 is basically the same thing, with a different viewfinder patch, etched frames lines as opposed to the selectable framelines of the SP. This camera was introduced in 1958, after the more professional version, the Nikon SP. Naturally, being a Nikon shooter, and having already fallen for the contax style, the Nikon was in my sights. The Nikon rangefinder was based somewhat on the Contax as well in that they have the same mount (slightly different) and the same focus wheel. Nikon S2 Call me a contrarian, but I’ve always preferred the austere classic S2 of 1955-1957 (which has a large 1.0x finder with a single, stationary reflected frame line for 50mm lenses) over the legendary Nikon SP of 1957-1960, which famously has dual viewfinders providing parallax-compensating frame lines for 50, 85, 105, and 135 mm lenses. I loved the design of this camera, and the photos is shot the camera, being a Russian copy, was built to Russian standards, and the film spacing was never consistant. I used to own a Kiev 4a, which was based (read exact same) on the Contax IIa. Having owned a few rangefinders prior to this, and having sold them for varying reasons, I started the search for another camera that I would be able to rely on.

Nikon s2 rangefinder review