Looks like 30s to early 50s gm bed Gms had this styling till around 53 the fender mounting area looks like an earlier version unless it’s been messed with
It's got a skin of sheet metal over the whole interior. It's full of garbage right now so can't really get a picture to show it.
The tail gate could be home made. The tailgate with the tubing down the center probably was not an auto factory item. The corner braces will probably give away the original manufacturer better then anything else, but its pretty obvious to me the bed has been modified, so who knows how much of what we see is original to any brand? This bed and tailgate are completely custom built. Other then the rear fenders, no real truck bed or bed parts were damaged in this pursuit.
Looks similar to the Studebaker bed I used on my chevy And how a stock 41 chevy stake pocket looks on the left (modified chevy on right), you can see the Studebaker is much tighter to bed side
Either way, most likely, that top tail gate "rail" would have been a round tube, and the bottom "rail" could have been a round tube, or a square tube with rounded corners (Dodge used this). The tailgates were often bent because heavy stuff was loaded onto them to be pushed into the bed, removing the tubes and replacing them with angle was pretty a standard procedure. Sometimes that angle was pretty thick stuff. The angle iron side "rails" were pretty standard from the auto companies.