Usually if not always a dump bed, and likewise the controls for it, were installed by the shop that sold and installed the body, not the GM factory. They may have had some instructions but mostly they installed that stuff how they wanted, which can be different than the body dealer/installer in a different state, different than a different brand of body, and different than the other mechanic in the same shop.
Having looked at a lot of old trucks I've seen lots of things done after the truck left the factory that if I had installed whatever it was I'd do it differently. Those levers won't be the last thing you see and scratch you head as to why. Often done the quicker easier way to just get it done and get the truck out the door.
