A.
Around the 6th Edition Restricted Articles Regulations, there were 4 codes for poisonous substances according to the degree of danger, I.e., RPA, RPB, RPC and RPD. RPD never made the DG ranks and was disregarded. Later, RPA promoted itself to RPG, and RPC was dropped because of low toxicity. Thus only RPB alone remained as code for poisonous (toxic) substances, and survived to this date. In the early days, only the United States had legislations on dangerous goods (hazardous material in U.S. Terms), and IATA was much following US rules. The US 49 CFR even carries today the term Hazard Zone A and Hazard Zone B for Division 6.1 substances.

As for the origin of the letter "S" in RSB, it comes from "Styrene" the substance from which "Polystyrene" is produced.
"Styrene" is an "Aromatic Hydrocarbon" . "Polystyrene Resin" which is made by polymerizing Styrene is a "thermoplastic resin" with equal importance as "synthetic resin" such as "polyethylene" and "polyvinyl chloride". "Polystyrene" is made into transparent containers, and "Foam Polystyrene" is used to make thermal food containers or buffer material. "Polystyrene" is also used as material for plastic models along with "ABS resin".

⟨Note⟩ "ABS resin" a co-polymerized resin of acrylonitrile, butadiene and styrene. The acronym is used and therefore the name "ABS resin".

Instead of the rather confined title of Restricted Styrene Beads (RSB), ICAO/IATA chose to use a broader title of Restricted Polymeric Beads to house other similar resins along with polystyrene.