First off, is this an original arm? That ring sitting on the cowl is plastic, and I can't remember seeing any others like it.
The arm has a tab underneath it, but I can't push or pull it to release the arm. The tab is in the second pic about the 10:00 position on the dental mirror (not a good pic, but the best I could do). I thought I better ask here before I mess something up, because I was starting to get frustrated messing with it.