Some ammeters have tangs (metal) or fingers (plastic mouldings) in the body, just under the bezel. The rubber ring goes under the bezel, and the ammeter pushes into the mounting hole. The tangs spring out to hold it in place.
Rubber ring acts as a seal and keeps the tangs pressing against the back of the mounting panel. Sometimes the mounting hole has cut outs, matching the tangs. The tangs pass through the cut outs, then rotating the ammeter slightly locks it in place.
A design using a metal bridge will have separate mounting points, this is the usual style for larger instruments, typically used on car dashboards. The terminals need to be kept well away from any metallic mounting.
Swarfy.