On 1937-55 1/2 and 3/4 ton rear bumpers there exists an unused center square bumper bolt hole that sometimes brings up questions from restorers. “Why does this hole exist and what is it purpose?” The answer relates to the attitude toward trucks during those years. They were for work and keeping their production cost low was a priority.
The bumpers during 1937-47 were the same front and rear. The center hole at the front held a vertical steel bracket which was needed if the truck was hand cranked. Rather than make a 4 hole rear bumper, GM simply used their front on the rear. Even in 1947-55 with a slightly different horizontal shape, the factory 5 hole punch was used on front and rear. Therefore, the rear bumper hole has no purpose. To cover this hole, GM produced a special bumper bolt that has become very rare. To save costs (it is a surprise that anything was used) GM created a one inch long stud held in place with a sheet metal speed nut. It has no threads and its head is covered with a stainless cap so it looks like the other bumper bolts from a distance.
Most of these original rare filler bolts will have dents and scrapes on the stainless cap. A skilled person can place a new stainless cover from a more common replacement bolt and make this rare unit look like new.