Gonna take a wild guess and say its a 172/182? Make sure the new disc have ABS rings in already.
It's a toothed ring near the centre of the disc, back surface iirc.
Have a look at them when you buy them or ask the parts man. And buy the ones with the wheel bearing already fitted, the chances are they will need doing as well. OE's are plenty good enough.
invoice is in the post!
hope you fitted the nobble on the back of the pads into the piston correctly!!! oh AND torqued the hub nuts (very important, or you could die)