and how do I get it off?
I'm changing rotors (et al.) on my 96 D1 but when I went to take one of the rear rotors off of the hub, I found this metal disk covering the heads of the bolts I need to get at. nothing similar on the fronts
looks like it might be something for the ABS sensors? I only say that because of the placement of those sensors in the rears as opposed to where they are on the fronts. just a guess though.
any advice is greatly appreciated, thanks in advance

 

That's the ABS Sensor Ring. Flip it over and you'll see 5 small nyloc nuts that hold it on to the rotor. Give them a shot of penetrating oil and remove the ring. Go easy because the studs can break. Have at it.