No, you are seeing the yellow brass under the copper plating. When people refer to mixed brass, they are referring to the brass color of the actual casting, not the plating.
Some pieces can be yellow or red brass under the copper or nickel plating.
You have to use chemicals & a 12 volt charge to remove the plating on both the nickel & copper. The plated registers cost extra so National made the normal bronze castings with a lot less copper in the mix which is yellow brass. Often there were parts that were not made in yellow brass & they just used the bronze but very seldom would they use all yellow or all bronze. The nickel machines had a solid nickel drawer front. The copper plating is a bit thicker than the nickel but glass beading won't get it all.
Mixed brass
