correct me if im wrong but i belive the only difference is that you order the open diff one if your car is an open diff prior to the purchase, and you buy a viscous if you have the Vlsd, does that answer your question??
s14 and s15 rear subframe are identical… the rear pumpkin between s14 and s15 should more or less be the same. s15 lsd is helical lsd and the speed sensor is at the diff, not on the tranny, that’s as far as i know