The placement of rear axle bumpers by GM on 1/2 tons proved to be an important feature. Owners can often load cargo over recommended weights, their shock absorbers may lose their resistance, and there is the existence of uneven road surfaces. All this can make axle bumpers very important.
During the hauling of freight, these bumpers occasionally stop metal to metal contact between the frame rails and the axle housing. GM placed them just above the rear axle.
1947-1953 1/2 ton (above)
In 1954 GM increased the depth of the 1/2 ton pickup bed from 15″ to 18″. To do this they lowered the frame rail arch above the rear axle. This shortage of space caused the bumper to be placed at the side of the frame but still above the axle.
1954 1/2 ton (above)