There could possibly be another explanation here. I was given a 1968 Supra in exchange for computer work I had done for a client. It also had a funny patina to it... but absolutely NO pitting! Turns out the "patina" was years of dust and smoke, which protected the finish! I cleaned that baby up and you can see the results!
So, it is possible that's what's going on here. You'd really have to see the drum in person to make a determination.
Mark