I think the shield may be tied to the other metal bits on the pot. When you mount the pot to the chassis, metal will touch metal, and you may be all taken care of.
To confirm this, mount the pot and measure the resistance between that metal sheet and the chassis. You could also check to see if the shield is simply internally connected to one of the ground lugs with your meter.
If the shield requires its own wire, just connect it to one of the input STP drain wires.
-PB