I would not expect a 'groove' in a throttle body, but a slight 'ledge' would be a normal part of the machining, and allows as complete an airflow shutoff as possible. The butterfly should not move past the ledge. Even though no fuel passes this way on first pass, vapors do, and so it is normal for a varnish to build up. That should be thoroughly cleaned off before you make any judgements about the smoothness of the throttle action.
I don't know the correct setting of the adjustment screw, but I would leave it where it is as a first course of action.