As mentioned, there were many different brands of aftermarket beds available. I've seen quite a few, some with the fancy pinstriping you referred to. I would also guess that your bed does have tongue and grooved floor boards. They were made to haul grain without having it leak out through cracks in the floor.
Also, it could be possible that the "open" part of the rack that you referred to could have been installed above the other part for hauling cattle, then folded down for hauling grain like
this one. Both my brother and a neighbor had those. I believe they called them combination boxes.
