Hy fmartinmn, that is the standard three speed from a 55B - 59 Chevy truck, or 55 - 57 Chev car, non syncro first gear, I believe it's called an SM318, hope that helps.
It is indeed an SM318, specifically, it's froma '55 Chevrolet or GMc truck. The levers and side cover are the givaway. The levers going in opposite directions were '55 only, '56 they went to both levers pointing down like the Passenger version. The shift shafts and side cover are also unique to this application. The entie side cover assembly will directly interchange with the Passenger and later Truck version.
Saginaw or Muncie? According to a GM transmission specialist I spoke with, it's both and neither. The SM318 was made at both plants. The "Saginaw" and "Muncie" designations came much later, when GM got into manufacturing their own 4 speed Passenger transmissions, as well as each plant having their own version of the 7 bolt side cover 3 speed.
