If you want to restore the truck you need the SN off the frame. Its usually above the passenger side tire on the frame... you may have to look for it because it can be anywhere on the frame arch above the tire. It should resemble CCKW 353 - XXXXXX - XX.
The part between the 353 and the - can tell you when the frame was manufactured (will let you know what the cab, brakes, engine, bed should be) and the part after the dash will tell you the body type and axle type (was is originally a dumper, an air compressor, fuel truck, cargo, etc).
As for the differences between an M series bed and a CCKW bed they are minor:
Notch for gas cap (depends on what type of bed).
8 inches to wide
Front side is a little different.
Tailgate is a little different
No tool box on rear sill
Sides have some cosmetic differences (taper on bottom channel)
Like I said minor issues. Some guys have actually made an M series bed into a CCKW bed (well, an 80% copy) and you would never know unless you nit picked.
