I have several of these modules, they all use the same tube so no worries there.
The cylindrical metal piece the tube screws into is electrically connected to the anode, but the actual heatsink isn't. It's actually at ground potential when the tube is in position. The brown coloured disk is some kind of insulator that's thermally conductive. Not sure what the material is... I thought it might be beryllium but its not the right colour.
