Pretty sure it's a nut, kind of like the back of a recessed brake? Maybe not? It has a large dome capped head. This looks to be all original equipment, the back half is black plastic, and it has a half round hinged chrome part that goes across the front. I have a can of plasti dip, believe it or not, lol. I will give it a try, thanks. maybe a strip of inner tube could do it too. With 35+ bikes around, I have my share of old tubes.,,,,BD