The way I see it on mine is the socket doesn't really hold the backrest, it is just an anchor point for positioning, the thumb screw (solid stainless steel about 1-1/2 long and 1" diameter finger hold) locks the backrest from being removed. If your thumb screw no longer threads into the body then tap it and order another dimension of thumb screw to fit. I ordered a new thumb screw on the internet from a machine shop cost me less than ten bucks. Don't worry about the holes behind the sink. let me know if that helps.