If something fell off inside the headlight, it should still be there. Did the headlight fall out? Since my whole headlight fell out and the clip had unscrewed and fill out with the headlight. I tried to find another one and ended up getting one from a vintage Triumph parts store. But it's real purpose isn't adjustment, although that is the secondary use of the slot, it's really used to keep the headlight in if you get the optional ones that have a separate trim ring ahead of the one that is one piece on the stock version.
After reading your response again, perhaps it doesn't do anything with the stock light?