Excellant point on availability - over analysis is a risk - but the analysis shows that, for a WWII fleet, steel 50 foot box car models are a fleet "gap". Bowser has some of the PRR fleet covered, but the only other Styrene car I have found (thanks to the members of this list), is the Proto 50' steel box cars.
Dave
Sunshine and Speedwitch now offer 50' resin kits, so that helps. And most 50' cars in your era were composite wood, and there are even more kits for that. The SP fleet of 50' steel cars in WWII was about 2,500 cars, a large fleet for that era -- 80% were double door automobile cars.
