Michell binding posts?

My 30 year old Proacs have Michell binding posts, looking a bit like their current 'Big Mother' but massive and nickel on the surface. I took a mini-file lightly to it, and still nickel. They are so heavy that I had to put a magnet up to them to be sure they weren't ferrous, but I assumed perhaps nickel over solid brass. But if that's the case, the nickel is mightly thick. Anyone know what they are actually made of, circa 90's?