If you're going by decals there's a bit of ambiguity.....
In a nutshell on construction though it goes a little something like this:
89-90: 1" HT: Double drops, round chain-stay and seat-stay bridge, thin stay tubes, drops welded on outside of stays only. Stamped S&M P on one of the rear drops (either side).
http://bmxmuseum.com/bikes/sandm/629561991: 1" HT: Double drops, flat single plate chain-stay bridge, double plate flat seat-stay bridge, thicker stay tubing, drops welded on outside of stays only. Start of the SMD stamping on the rear drops (either side).
See the chrome one in me thread here:
http://www.radbmx.co.uk/forum/index.php/topic,165202.0.html1992- early 93: 1" HT: Double drops, round chain-stay bridge, double plate flat seat-stay bridge, thicker stay tubing, drops welded on outside of stays only. SMD stamped. The platform DB of mine is one of these. Optional AD lugs available.
Late 93 - 96: 1" HT: Solid drops, round seat and chain-stay bridges, thicker stay tubing, drops welded both sides of stays. SMD stamped. Optional AD lugs available, by later 94 though most came standard with AD lugs.
Again see the non-painted frame in my thread here:
http://www.radbmx.co.uk/forum/index.php/topic,165202.0.html95 - 98: 'Oversized' Dirt Bike: 1 1/8" HT: Solid drops, round seat and chain-stay bridges, thicker overall tubing, drops welded both sides of stays. SMD stamped. All with AD lugs.
http://bmxmuseum.com/bikes/sandm/174061997 - 2002: Dirt Bike 'Next Generation': same as 95-97 but with a wrap around down-tube to HT gusset, and enlarged free-wheel protecting rear drops, chain-stay wishbone:
http://bmxmuseum.com/bikes/sandm/565761998 - 2002ish: Dirt Bike 'Classic': Same as 95-98 but with a 'gusset plate' on the chain-stay - bottom bracket. Some have a wrap-around gusset on the headtube between the top tube and down tube:
http://bmxmuseum.com/bikes/sandm/753322002 - 2004ish: Same as The 'Classic' above but with seat-stay AD lugs:
http://bmxmuseum.com/bikes/sandm/32587Late 2003+ starts the move to New School with a change in geo etc.
Point worth noting though is there's anomalies (especially in the early ones) and I'm probably wrong