The ICC ruling on billboard reefers was done for the benefit of the railroads. What was happening was let's say Hormel told a railroad they needed six empty reefers at their plant tomorrow. Next day six reefers would arrive - lettered "ARMOUR" in huge letters, because they were the first available meat reefers the railroad had on the property. Hormel refuses to ship their products in the cars, essentially don't want to pay their money for a rolling advertisement for their competitor !! So the railroad has to try to find six more reefers Hormel will accept.
Eventually the ICC said, OK you can only letter a car with letters x" high etc. so the name isn't as prominent - and shippers can't refuse cars based on what the car's lettering is, as long as the car will serve the purpose they have to use it.
BTW in in the 20''s - 30's many (most??) new cars (boxcars, reefers, hoppers etc.) were built entirely of steel but due to wartime shortages went back to woodsides in WW2 (steel being needed for tanks, planes etc.) so in the fifties there would still be quite a few cars similar to the Athearn woodside car still around from the war years.
I believe the last iced reefer to run in the US was about 1973.