the difference in the shafts are: A threaded shaft has threads on the crankshaft all the way down, this is what the clutch nut screws on to. On this type of engine the clutch nut has a shaft that comes off of it that the clutchbell bearings ride on.
An SG shaft only has a few threads on the crank, just enough for the clutch nut to screw on. Past the threads the crank shaft is round and this is where the clutchbell bearings ride.
Just look at each crank shaft closely and you will see the difference