On various receptacles I found a self-grounding clip. I am not completely sure about its funktion.
From what I can see, there is no plastic screw holder any more between receptacle box and grounded metal, because the self grounding clip protects the screw from falling out.
So there is enough contact between (metal) box and recpetacle which means if the box is grounded, the receptacle itself is grounded (or the other way round).
Is the clip also suitable for grounding metal cover plates or is this done via the grounded screw thread for cover plates?
The "self-grounding clip" is an agency approved device designed to automatically ground the receptacle to a grounded metal box without having to connect a ground wire to the receptacle. That is, if the ground wire or metal conduit is secured to a metal box, this type of receptacle will be "self-grounding" without an added ground wire.
The "self-grounding clip" clinches against one of the screws securing the receptacle to the box.