I reference those sheets a bit. Here's something else cool from there.
1984 Chevrolet Pickup Truck Brochure
A lever lock is used to lock the rear brakes. When released the brakes work normally. When in lock, it acts as a 1-way valve holding the rear brakes on after the petal is released. So to use it flip it to lock and press the brake petal. Once the petal is released the rear brakes should stay on. To take it off, push the brakes and flip it to release. If every thing is working the brakes should release once you stop pressing the petal.
There normally used as an extra safety device. Like wreckers, brush trucks and on trucks with a boom lift. There are probably other uses but those are the ones I know of. Also being from MN. My guess is it was used with gravity boxes(grain carts), for hold the truck well loading or when waiting inline at the elevator. With out knowing anything about the truck, it just gives off a farm truck vibe. As I'm not sure why else you have one.