This kit car came out in the 1990s. It is a one-piece metal casting (trucks and couplers not included). I'm told that this model was copied from Arnold's 86' flat car (just with the sides modified).
I don't know what specific prototype this model might be based on (if any), but generally speaking, flat cars like these first entered service in the 1960's. They were designed to accommodate the latest 40' trailers that were just being introduced on the nation's highways. Each flat car was set up to transport two of the trailers. Subsequently, these dedicated TOFC (Trailer On Flat Car) flatcars became the defacto standard for intermodal service during this era.