In 1952 my father purchased a new IHC L160 with a dealer-installed GarWood cam-and-roller hoist. That design, like this 1918 one, could, and occasionally did, allow the contents of the box to slide toward the back of the truck, which caused the box to flip up as far as the safety chain allowed, which was enough to spill grain over the back side of the box. Messy and scary! Later versions of that make of hoist had a revised design which prevented the hoist from lifting the cam off the roller.

The good old days ...

I'm not so sure. I think that the rig is to unlock the tailgate when dumping a load. When the bed is raised, the unlocked gate would open automatically because of gravity (and closed similarly when the bed came back down). I've seen these on many modern-day dump trucks.

An arrangement of lever, pulleys and wire rope appears designed to automatically lift the gate as the bed tilts