u can also kind of see a definite seam around the hub centre casting, as if its pushed through the outer drum section

my guess would be they have been on so long that they have simply welded themselves together, a few taps with a hammer should release it if it is

would explain a hell of a lot though, and im looking through old pics of the back of my old 1 piece drums and the hub looking part isnt as obvious, its cast into the drum more subtly without a definitive round outer to the hub section