1948 through 1954 use the same case with the same casting number. However in about 1952 they changed from a 2 lug to a 4 lug synchronizer, changed second gear and a few other things. You have to be careful when you rebuild one of these. The entire set of gears will move from one case to another, but you cannot mix and match other parts. The early and late is easily identifiable by pulling out the input shaft. If you expose a 2 lug synchro, you have the early model. If you expose a 4 lug synchro, you have the late model.
The HD transmission is a Borg Warner T89 which is very similar to what Ford used behind Y-Blocks and their FE engines.
The hardest part to find for the old 3 speeds is the synchro. At one time the brass rings were reproduced in the aftermarket. but the brass rings as well as the complete synchro is getting really hard to find.
