The item shown in the mirror is a vent, leave it alone.

 

Here is a picture of the light as it came from the factory with the tamper proof caps still in place and the vent under the high beam. The motor on your light is mounted in the center between the high and low beam. You can easily see both the adjustment gears on the edge of the housing. Those move their respective light laterally.

 

Here is a picture with the tamper caps removed and the also the vent removed. Lights manufactured later in the model years did not have a vent. The later models were capped.

 

Brilliant! Yes, I finally managed to find the second horizontal adjuster - impossible to see without the aid of a mirror, but I can feel it. It's a good job that I don't need to adjust that one! I tested the dashboard headlight leveller - it works for both headlights, but only for the last, final position (0,1,2,3), when the headlights drop by a few degrees. So that works too. I then turned my attention to the vertical adjuster, at the top of the headlight leveller motor. It appears that turning that Allen socket anti-clockwise raises the beam and clockwise lowers the beam. I turned it anti-clockwise as far as it would go, until it "clicked". It raised the beam a little bit, but not really by very much. I think it was already adjusted to the highest pointing angle. So I then went to the driver's side headlight and lowered that beam by turning the corresponding adjuster clockwise. That worked very well. Both driver and passenger beams are now more closely aligned. Not perfect but I will have to take the car out at night for a full test. I must confess that I've never actually driven it at night......!!!! THANK YOU!