Dont know what month, but generally speaking 89-90-91 is S1 and 92-93 is S2.
For some useless trivia: S1 and S2 have different headlights, front bumper (GTS), instrument cluster, gear/radio surround, hoodlining material, carpet, seat trim, different cam cover (RB20), different size bolts here and there. Apparantly tail lights are different too, but I cant confirm.
Dohmar, the "S" on the GTSt steering wheel is the Skyline logo.