Casey,
You see those hatches appear on a number of C&S box cars.
As nearly as the railfan community can figure out, they were used to load Moly at Climax like an early covered hopper. There are photos in the CRRM rail annual that featured the South Park Lines that were taken from a Black & White 16mm movie shot by Bruce Triplett. There never seemed to be any sort of "Special" numbering system or designation for these cars, but one of the Triplett movies shows at least three or more in one train.
Harry Brunk picked up on these cars as well and modeled them.
Rick