This is the external plastic cup, on the back of the mirror. It houses the screw that holds the mirror arm to the internal parts, that allow the mirror housing to swivel. If you are fitting one of our mirror arms or replacing the internal swivel parts, (click on "Related Products" above, for info), you may also need this cup. This is an original Geco part. (Along with Cipa, Geco made original equipment mirrors for many of the French car and motorcycle manufacturers).