You're talking about the part that goes across the rear of the bed and attaches to both bedsides, correct? If so, that is the rear cross sill. All of these old trucks have them , even the non-'Bolt ones. Sounds like someone made a home made bumper and just mounted it to the rear of the bed than making brackets to mount it correctly. The part is available new, and you do need one as it hold the rear of the bed together and it's where the wood bolts down to at the back of the bed. Welding in the new one isn't 100% necessary as it is bolted as well.
