I can't wrap my head around that 3 speed case having 4 forward gear in it. Are you certain that it has 4 forward gear--- as in you have driven this truck and actually used all 4 forward gears? Is it possible that something is broken internally and the mainshaft is binding on itself and giving the illusion of a 4th gear? I had my modern 5 speed Cummins powered transmission stick on me and act as if I had extra gears. It turned out to be the pilot bearing in the flywheel would bind sometimes and cause the input shaft to turn when it shouldn't be. I know that is a long shot with your 1946 3 speed case transmission, but I'm curious.
Also, on a totally different note, I've seen Chevrolet literature advertising heavy duty and light duty 4 speeds in the 1940's 3/4 tons. I wonder if this is one of those fabled light duty 4 speeds that I never seen in person? They did have light duty and heavy duty 3/4 tons for a while, so maybe.
