It was not a remodeled S2,but rather was designed concurrently with the Nikon F, and has more in common with it than the S2. It was every bit the improvment on the S2 as the S2 was on the S.
Visually, the most distinctive feature of the camera is the long finder window on the front. This unique finder had projected frame lines that were selected by a dial on the top of the camera for lenses of 50, 85, 105 and 135mm length. The combined rangefinder/viewfinder is 1:1, and has a second finder for the 28mm lens, with an internal frame for the 35.
The shutter was comletely redesigned in this camera. It featured a single dial with speeds from 1/1000th to 1 sec, plus bulb and time. It did not spin like previous shutter speed dials, and had teeth on it to allow it to couple to a shoe mounted exposure meter. In 1959, the shutter curtains were made of titanium foil, replacing the cloth shutters found on earlier cameras.
Other changes were the addition of a self timer to the front, and the inclusion of a motor lug, allowing any SP to be motorized with minor modification. This was also the first Nikon to be generally available in black finish.
As I stated earlier, this camera was developed concurrently with the Nikon F. Insistance on making the F and SP share the same frame for finacial reasons caused delay in completion of the F. In September 1957, the SP was introduced. Although it was enthusiastically recieved, and became very popular, it was overshadowed by it's brother, the Nikon F, which was released in 1959. As a result, only 22,348 cameras were produced.