As I understand it, the issue had to do with income from advertising which had the effect of defacto lower shipping rates, thus getting around regulated tariffs. Railroads and their other customers objected and the end result was all billboard advertising was eventually banned by the ICC. By the end of 1937 virtually all billboard reefers were gone. There's a ton of info on the Internet if you'd like more detailed explanation, but for modeling purposes, I believe this to be accurate.
Have fun, Otto K.