I would think they would haul 'traditional commodities" handled by RBs, ie things that need to be protected from hot or cold. Canned and glass goods, beer (other than unpasteurized draft beer), some types of produce, etc. Or things that need extra protection, like bagged flour, I don't have a equipment register from the 1940s but I bet there are a bunch of RB-like cars in there (not sure if that was the designation back then).
Best wishes, Dave