Although most of the early S&M frames changed from week to week (in small ways), there were only 2 main types of 1" dirtbike. The mk1 never came with 990 mounts and had a much thinner rear end. The mk2 had thicker seat and chain stays, and came with or without 990s, later they pretty much all came with them.
This frame looks to be a late mk2, probably 94ish.
As I have just said on the other thread, any chance of swapping the blue stem for a different redneck? Ive been after a blue one for a while.