My guess would be cast aluminum. Cast zinc would be very fragile. Just a guess though, without one to examine. If anyone has a broken one, the way to tell would be the crystalline grain structure seen on the broken edge. Die-cast would have big unevenly sized "crystally" looking grains. The grain structure of aluminum is much much smaller and more even in nature.
I wonder if the engineers at Savage were arguing among themselves 100+ years ago over the best material to use for the rotors. I'm glad brass won out.
Those rotors must have been tricky buggers to cast. Obviously done via the lost wax method of casting, as sand castings would be a bugger to get nice and smooth/shiny over all the surfaces.
