Magnets don't stick to aluminum shells either, a "magnet test" is useless in this instance (or on any Supra).
I doubt it's brass but the only way to tell is to pull off a lug and file one of the holes a bit and see what color the shavings are. The interior color doesn't tell you anything because they plate the drum with copper before the nickel before the chrome (which is really more of a protective clear finish for the silver nickel plate) and these can give the interior a brass like hue...which has fooled a LOT of people.
It would sure be cool if it did turn out to be brass though.