Looks alot like the Chevrolet "Unit Design" Bodies from the mid 50's through the early '60's. Especially, since it has the two ribs, in the rub rail. The earler G.M. "Unit Design" Flat, & Stake bodies had a smooth slight curvatured rub rail.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers