I do not recall exactly when but around the 6th Edition DGR, there were three Cargo IMP codes for Poisonous substance, namely, RPA, RPB and RPC. “R” stood for Restricted Articles, “P” for Posion, and “A”, “B” and “C” indicated the degree of poison with “A” being the highest. Later, at one time, “RPA” was elevated to Poisonous Gas (RPG) and “RPC” was dropped as the poisonous level was lower than standards. As result, “RPB” was left as the Cargo IMP code for Poison (now Toxic substances) and being used to date.
There is another peculiar Cargo IMP Code “AVI” which stands for Live Animals. In the early stage of aviation, France was the leading nation and all aviation treaties, such as the Paris Convention and the Warsaw Convention were written in French. After WW II, the United States became the leading aviation nation, and the Chicago Convention for instance is written in English. “AVI” is the only Code that still remains in memory of the then leading aviation nation, France. “AVI” originates from the starting letters from the French word “Animaux Vivants” (Live Animals). If American language was used, the code may have been “LIV”.