Those Crew Cab Conversions were offered by the Dealerships, when prospective buyers needed extra from their purchases. They were not Factory Assembly Line Built. The Dealerships, would place the orders with Contracted Outside Companies. These Companies, along with other non Factory add-ons, would be listed, and pictured (as with the picture Tim shows) in The Cheverolet Silver Books, which listed all available equipment, and bodies, availble for you new Chevrolet Truck chassis. The picture below, is a 1954 Crew Cab Conversion, I found a friend of mine.
Spanky Hardy Collector Of Fine Old G.M. COE Trucks & Antique Holmes Wreckers