The grey buttplates are an alloy of zinc and a few other metals meant to be a cheaper alternative to brass (which was needed for cartridges). It's called Zamac and was generally cast by Savage's primary buttplate subcontractor, Ilco (of lock set fame).
Zamac is an alloy of zinc, aluminum, magnesium and copper. Depending on the proportions and final intended usages, this alloy may be ZAMAC 2, 3, 5, 7, 8, 12, 27 and Tonsul. I'm not sure which specific formulation was used in Enfield buttplates.
Today, Zamac is mostly used for automotive parts, household appliances, toys, racks, carburetors, automotive oil pumps, automotive handles, plates or logos, articles for bath and cosmetics, ventilator frames, computers, photographic camera parts, refrigerator handles, regulators for gas tanks, hardware, buckles, ornaments, candelabras, buttons and trophies.
Hope that helps!