Depends on the period of manufacture. Ca 1910-1920 are brass with a gold plating. 1920 +/- to current manufacture are brass with a nickle plating with an exception of ca. 1948-1952 which are nickle plated plastic. i clean them with metal polish like Mother's and the end of a Q-Tip. Be careful with the plastic ones a they will be damaged by many organic solvents such as acetone.
Actually c. 1929, since from 1920-29 there was no medallion on walnut stocks.1920 +/-
And let's not forget the rare steel medallions from roughly the same period.to current manufacture are brass with a nickle plating with an exception of ca. 1948-1952 which are nickle plated plastic.