That could be the meter needle adjustment screw used to position the needle at zero and to match the other one. I have used it to match needle resting points on reel to reel decks before.
McIntosh Meter
Finally changed the glass on a McInosh MC7200. While working on it the black plastic screw on the face of the meter fall out. What does this screw do? On my other 3 meters with the power off the needles rest at -60 or a hair above. The one where the screw fell out rests at -52. Do not know where it rested before the screw fell out.