Leica Elmar 50 mm f/2.5 collar - collier d'objectif

This collar is designed to help the Leica Elmar 50 mm f/2.5 lens from fully collapsing inside the camera. This is necessary when the lens is mounted on Voigtländer cameras as these cameras are thinner : the lens barrel rear can damage the shutter.
This collar may be left mounted on the lens barrel even when used.