The cam 34 is customarily formed of a polymer such as polystyrene or the like, and its inner surface defines one or two shallow helical grooves 36, as shown in FIGS. 1, 2, 4 and 7.Shell 33 is often termed an A-shell in the industry, and it extends distally beyond cam 34, as clearly shown in FIGS. 1, 3, 4 and 5.