When billboard reefers were outlawed, there was an exception. If a car carried the product advertised on the sides, billboards wre legal. Since the meat packers were mostly private fleets and not owned by the railroads, the meat reefers continued to be billboard cars until the demise of meat reefers. Part of the reason (perhaps the main reason) that they were private fleets is that the meat packing houses were built with door spacing to fit 36 foot cars. When the railroads went to 40 & 50 foot cars, the meat packers were unwilling to change their buildings to use the new larger cars, so they had private fleets of 36 foot cars that were used into the early 1960's. Therefore you can be prototypical running 36 foot reefers decorated for Rath, Morrell, Hormel, Armour, or Swift. I'm probably forgetting some companies, but those are the ones I remember from off the "top of my head."