Well, I was playing with my motor last night- had to re-tap the front mounting plate since most of the holes where clogged with... crap. Here are a couple photos of the old rotor:
Front
Back
The newer sintered rotors have a shinyer finish, and the thin metal 'jacket' around the magnet overlaps the edges, right upto the rotor shaft almost- the old jacket clearly doesnt, and just wraps around the diameter of the magnet.
You can compare your magnets/rotors just by removing the front endbell- no need to pull out the rotor since its a nightmare to get back in....