The rubber quality on Pre-World War II vehicles was fair at best. Its useful life was limited. Thus, GM engineers did not add rubber pedal pads to early trucks. They were aware of the heavy abuse so many commercial vehicles would receive. In an era of limited income, GM knew few truck owners would never replace their pedal pads.
The solution was to add rows of bumps on the pedal surface. The leather shoe soles of the driver then prevented his foot from sliding across the pedal while driving. These bumps usually outlasted the life of the truck.