Rock Island Flat car ca. 1931

[Puddlejumper]

I am going to model a load of John Deere tractors circa 1931.  The factory is in Waterloo, Iowa and the Rock Island was the rail service provider.  Can you guys help me out with what type of flat car would be most correct for the year 1931 in HO scale?  I was thinking the Athearn car is too modern, maybe something pre cast frame, with arch bar trucks?  40 foot?  Or were cast flat cars around by then?  I am having trouble finding much info or photos of prototype Rock Island flat cars.

Thanks.
Dave