It is very seldom that Chevrolet parts books refer to a casting number. However in this case, the parts book clearly states that a 591195 casting number is a 3 speed transmission 41-47 Commercial truck. The 1953 edition of the Hollander states the same thing.

https://plus.google.com/u/0/photos/...4758965010&oid=112636930370126476644

I find it rather hard to believe that someone put 4 speed gears in a 3 speed case. There is just not enough room in the small 3 speed case.