Beyond the scope of this list but . . . these cars relate to the subject line and the cars have built dates prior to 1960.
In 1965 Canadian National modified 40-foot boxcars with roof hatches for potash loading (Melville Sask). These cars were routed to smaller co-ops and elevators (including in the US) that could not handle the larger 100-ton covered hoppers. The rectangular hatches were underneath the running boards second panel in from the ends.
CN 440000-440199: 10-0 IH cars, NSC-2 ends, rectangular panel roof, 6-foot doors, blt 1945
Photo on p. 6 of Canadian Rail Car Pictorial, Vol 2 A, by Richard Yaremko
CN 442000-442099: 10-6 IH cars, NSC-3 ends, combination roof*, 8-foot doors, blt 1957
Photo on p. 26 of Canadian Rail Car Pictorial, Vol 2 A, by Richard Yaremko
*Interesting looking roof: center panels NSC style (look like PS bowtie panels), second panel in from the ends rectangular shape and have hatches under the running board, outermost panels blank. Elgin Car Shops (Yarmouth) makes a model of the unmodified version of this car.
Captions in the books indicate that these car were in service 1965-1990.