Most likely, the trans pictured was the SM318 with the Borg Warner overdrive that's in my own '55 1/2 ton. The overdrive isn't so much of an add on as it is a slightly different transmission. The main transmission case has a few differences from the standard 3 speed, mainly a shorter mainshaft, extra oil return holes, and a provision for the reverse lockout rod. However, that trans is a direct bolt in for the standard 3 speed, and should work just fine in your application.
