Hurray for Google...
Pewter doesn't rust, so it's pretty safe to say that it's not that. Nor does nickel in EPNS.
Chromed or (non-silver) plated steel doesn't seem to have red rust - I have a late Victorian silver-handled stationary set that I had a quick rummage for, and the steel instrument parts of the set have a black oxide instead, and with only a slight texture. Nor does the steel shine when polished (although the remaining plating does).
So it would seem that the base metal is another iron-based alloy (possibly chromium).