Yes, it is probably original, however, not stock.
When the fleet side trucks came out i '58, people complained there was no place to stand while working from the side of the truck. G.M> made these inserts to answer that problem.
P/N988423 had a dealer cost of less than $6, and a list price of $10.
They were made of sturdy cast alluminum, and highly buffed and polished face. They came with mounting hardware and a template to show where to install on the badside with no sheet metal damage or warping.
There is reported to be plenty still in the dealership networks.
