the plastic hulls didn't really get rolling until the late 1950's,

DuPont (Remington & Peters) began offering their plastic SP shells in 12-gauge in late 1960. By 1962 they had added 10-, 16- and 20-gauge plastic shells. The Olins (Winchester & Western) introduced their Super-Speed and Super-X in "Compression-Formed" and their Ranger and Xpert in "Poly-Formed" in 12-, 16- and 20-gauges in 1964.