Here are some I picked up from orders:

KNH -- Knurled Handle (model 18 only)
NS/RDH -- Nickel Silver Reduced Hilt
STG -- Stag (handle material)
BM -- Black Micarta (handle material)
GM -- Green Micarta (handle material)
THU -- Thuya Burl (handle material)
WA -- Walnut (handle material)
SFG -- Single Finger Grip (handle shape)
FG14 -- Finger Grip #14 style (handle shape)
#2 -- Commando (handle shape)
BTB -- Brass Butt Plate (Trailblazer, M27)
DB -- Duralumin Butt (Cap)
NSB -- Nickel Silver Butt (Cap)
CI -- Compass
NHS -- No Hone Sheath
MCB -- Model C Sheath, black
MAB -- Model A Sheath, black

Not a comprehensive list, I haven't ordered every option (yet).