Just in case, you have an interest in the Advance Design original brake system, this might catch your attention.
For ½ and ¾ tons, in the left corner of the cab floor, is the foot operated park brake. Here, the lever extends vertically up through the floor so the driver’s foot can lock the rear brakes when keeping the truck in a stopped position.
To prevent outside air from entering the cab GM used a rectangle boot cover preventing any leaks. To secure the boot, a metal plate with a long center slot attaches to the floor. The attached photo shows this as GM installed it.
A different metal plate is required when there is no foot brake. On 1 through 2 ton trucks the total hole is covered with the same plate but no center hole. Interesting! It actually then becomes a just blank-out plate to cover the now unused hole.