Started stripping this baby down and underneath the bottom rolled edge of the drum there is a bright brass color showing through
The shell likely has a layer of copper, nickel then chrome. I think what you are seeing is the copper plating. Mine looks similar.
If you don't mind me asking, which Michigan GC did you buy it from? I try to keep an eye out for deals, since I live near one and work near another. Not long ago, I noticed a 50s Leedy duco snare listed at $100. It was gone when I got their a few hours later, shipped to Pittsburg.
Great snare - great price.