The U.S. Chemical rounds from WWI did not use gas bottles. The typical calibers were the 75mm for the French 75 and the 4.7 inch gun. U.S. Gas projectiles up to the 1950s use a pipe threaded adapter at the nose of the shell, that contains the bursting charge separate from the chemical. The main body of the projo is filled with chemical and kept sealed by the tapered pipe threaded adapter. The Adapter is very visible as it is hex shaped like a large nut. The design only changed slightly post WWII for the 105mm, 155mm, and 8 inch rounds that were in inventory up through the 90s. Those rounds had the same type of seal, but the large external hex nut shape went away, making the chem rounds harder to identify. To complicate the matter the U.S. Model numbers stamped in the projo body, for white phosphorus and Mustard gas projectiles in 105H and 155H is the same number. It might be acceptable to blow a WP dud round in place, but people really frown on scattering Mustard gas around an area these days.
Since the chemical had contact with the metal body of the projo, there would be a danger of residual chemical inside the body of the projo on the metal surfaces, if it hasn't been "washed" out by cleaning methods.