Now I say adjustable but damned if I can get it to move. You can get the nub (nub?) out of that hole, but that's about it.
But heads up - when I googled this (because it's the first I've ever seen) every other one is silver. So, maybe this is unmarked silver too? Maybe. It is so shiny and it is intricately carved, so I love it anyway.
Measurements: 2-1/2" inner diameter and just under 1" wide