Mystery gasket

[johnboy]

Recently removed the Halls sensor on the way to replacing timing chain cover gasket on 92 100GS. When can was pulled off, what appears to be a small (3/4X 1/2" ) rubber gasket fell to the floor. I have been unable to identify where this thing goes. I have studied the area around the sensor and cannot see need for gasket. The Clymer manual, has far as I can tell, does not show it. Could this rubber piece be some sort of damper between the can and the camshaft coupling?