1942 Chevrolet G506 WWII Truck

This is a "mystery part" on the underside of the dump bed on a 1942 Chevrolet G506 truck. There are four of these on the bed, one in each corner. The metal is held on with a small screw, affixed with a square nut on the top side outer rails. There seem to be some remnants of canvas strapping. I don't know if this was original to the bed or something someone added later.