The granny gear 4 speed standard from 1947 (I believe) until 1967 is called an SM420.
The tail shaft changed up a bit to fit torque tube drive trains and heavier trucks.
After 1954, the tail shafts had open yolks and driveshafts. These trannies are not hard to find, the cases changed a bit over the years, and a reverse light switch was added in '67.
SM stands for syncromesh.
Thunder54
