Some beginners tend to place 1936-46 cabs in the same category. Don’t do this! The 1936-38 and 1939-46 are a totally different design. Very little interchanges. The early style provided excellent building blocks for the new design 1939-46 trucks.
One major difference (when viewing a base cab) is the placement of the bottom seat cushion adjusters. On the early design a three prong bracket for a seat adjustment is attached in two places to the back of the cab. See Photo.
The new 1939-46 design gives a totally different way the lower cushion adjusts. It sits on four front to back above the gas tank strips. Two of these have small pegs which fit into holes in the cushion bottom. In this way the cushion can be lifted at the front and moved forward or backward.
NOTE: On both body designs the lower and upper cushions connect where they meet. Thus, at least the lower part of the back will move with the lower cushion. Unfortunately, your shoulders and arms will always be same length from the steering wheel.
Seat belts? Unheard of in the 1930’s and 1940’s.