Mechanical components on trucks were usually kept for many years by GM. Unless an improvement was needed, there was no need to change a proven design.
An excellent example of this is the rear ½ ton axle bumper. The design was used from 1929 through 1946 on Chevrolet and GMC ½ tons. A rubber bumper is held down on the rear axle housing by a metal cover with two ears. These ears are firmly secured by the two u-bolts that connect the leaf spring to the round axle housing. If the truck is overloaded or the shock absorbers are worn, the rubber bumper prevents metal to metal contact between the axle and frame rail.
Two of the attached photos show an original used retainer with bumper in place. The black bumper (now reproduced) is how the rubber part looks when new.