Check out this FEDERAL SPECIFICATION L-P-416 - POLYSTYRENE (GPPS) type material. The date on this is from May 9, 1949. You won't find this FEDERAL spec easily anywhere else. This Spec is a scanned in PDF format, ready for immediate download after purchase. There are 7 Pages of this spec. Please hit me up for any questions you may have.
PLASTICS, POLYSTYRENE, MOLDED
1.1 Scope-This specification covers molded polystyrene products. The chief characteristics of polystyrene molded products are their exceptionally good electrical properties which remain practically constant over a wide range of frequencies, temperatures, and humidities, excellent resistance to corrosive chemicals and water, dimensional stability, brilliant clarity, low unit weight, stability at low temperatures, and freedom from taste and odor. Metal inserts may cause cracking with temperatures changes because of differences in the coefficients of expansion of the metal and polystyrene. The material is not suitable for uses involving continuous exposure to temperatures in excess of 150°F