[Bob Putnam]

Chad,

The ring is an anti-squeal damper that's used on some Jag models. I haven't seen any aftermarket rotors that use it, and with the right (Lucas/Girling) pads, we don't get squeal anyway.

The Jag manual doesn't give a minimum thickness for the rotor. They simply say
"Replace the rotor if it is not smooth or has ridges." They're not expensive.