This component is on the LH bank (n/s) of my 1991 500SL (R129). It seems to have an electrical connection and a thin vacuum pipe going to the underside. As you can see, it sits above the injectors on that side. The black 'knob' or blank rotates but doesn't appear to do anything. There is a graphic on the front of the bracket with '1', '2', '3' in little circles that are connected by lines. Just curious to find out really.