Here is the way I see it. Up until 1950 the carrier slide down a slanted channel that ran parallel to the chassis of the truck. The sliding unit consisted of 2 pieces that looked like a big "U" mounted 90 degrees to each other. The end which dropped was supported in position by a "J" bolt in a hole in the rear sheet metal of the box.
After 1950 the unit was different in shape with the tongue having a flat spade-like shape and was dropped down at the right rear corner of the chassis. It was also held in place by a "J" bolt connected to the corner.
There are illustrations in the Chevrolet Assembly Manual.
