It looks like it may be a "composite" rotor. The hat is steel and the friction area is cast iron. Ford had a huge recall on such rotors on the T-bird and Taurus back about 1991 because they sometimes would come apart when subjected to salt rust. The design was a way to make the rotor lighter during the push to improve fuel economy where every ounce was questioned. The composite design never was one that I much cared for. They were even hard to properly machine because the steel portion would flex enough in the lathe (without the support of the car's hub) to make the tool bit chatter and make an uneven cut.
Drilling that part of the rotor, composite or not, was insanity. I'm glad I wasn't driving that car when it broke loose.
