The SS package was available on 88 models only. It consisted of pearl white paint, white wheels, recaro seats, a viscous LSD instead of the standard clutch LSD, a lower air dam in front, and it lacked the adjustable shock absorbers that were standard on other turbos. There were no options avilable on the ss, they all came with a manual transmission, analog dash, t tops, and a mostly black interior. If a car is different than this then it is not an SS model.
Lacking the digital dash and fancy radio, power seats, and adjustable shocks they are lighter than your normal 87-89 turbo. I'm just guessing here, but figure 100 pounds are saved over a similar non-SS turbo car. The SS is commonly called the SE, but S/S is the correct designation according to Nissan.
The SS is the rarest Z31 made Compare this to the 84 Anniversary Edition production of 5464 or the 1996 300ZX production of approximately 2750.
The SS recaro seats:
A nice 100% mint condition SS at the National Z Convention in 1997(98 maybe?):
Good side view: