I have a 1940 GMC AC503 (3 1/2 ton). The truck has a Clark 202V Tranny. This is the original tranny to the truck. It is a 5 speed with the reverse being to the right and down. There is no lock for reverse. You can see pictures in my webshots page. Look in the teardown album.

http://community.webshots.com/user/bda600