The autos were loaded on the open racks just for the commercial, as normally they would not have the hubcaps installed until dealer prep time.( Hubcaps were and are shipped in the car trunk.) Also quite unlikely so many different makes of only Plymouths, from many different assembly plants, would end up on the same auto rack. MOPAR production plants would normally make Plymouths, Dodges, and Chryslers, all on one assembly line, where the cars shared common platforms and mechanical bits.
The truly amazing thing about the commercial, however, is finding two Penn Central units that are clean and running!!!