This car would have been assigned to/shipped from the Westvaco chloralkali facility at South Charleston, WV. That facility began life as the Warner-Klipstein Chemical Company around 1915, was acquired by Westvaco in the very early 1920s, and later by FMC (Food Machinery & Chemical Corp.) in 1948.
I've seen other photos of cars in this dual WestVaCo/FMC scheme in the past, but I'm drawing a blank as to where, and don't have any saved off in my project folder for Carbide/FMC. I'll dredge through my dead-tree notes sometime this week and see if something surfaces. It's entirely likely the photos I'm thinking of were in regional history books. Not likely to be better quality, but might at least be enough to get reporting marks.
I _think_ these cars were leased SHPX cars, but I could be confusing them with the rest of the leased FMC cars. Models (and possibly decals) have been produced for cars assigned to both the South Charleston (inorganic) and Nitro (organic) WV divisions of FMC, but I've not seen any mass-produced models in this particular transitional scheme.
Incidentally, if you (or anyone) produce decals for these cars I would be greatly interested in a set or three. My long-term modeling plans include the C&O yard which directly serviced this facility and possibly part of the facility itself.