Well, the real experts will be aboard soon, giving you good information. But as a relative newcomer, I'll guess that you're dealing with a 2 ton truck. The duals will be factory, but I suspect these were shipped from the factory as a cab and chassis, and an aftermarket shop put on the wooden bed later.
Or possibly a farmer built the box himself to haul grain.
The question of whether the box is "original" is not straightforward. Did it come from the factory? No. But it was probably added soon, either before the first owner got it, or soon thereafter. It wouldn't have been much use to him otherwise.
So while not "Factory", it was certainly "Original" in that the original owner probably set it up this way.
