I made this lamp for my son out of his old Miyata he used to ride all over on.
It was too good a machine to just sit outside and ruin. We never noticed this "spacer" lever on the front brake lever. When swung out, it adds some slack to the brake adjustment. What's it for?
Question is specifically about the silvery lever outlined in Red, not the mounting stud outlined in Green.
Additional photos to show other parts of the bike for relevance: