I would like to comment on James Hooker's Osram Miniature UV-A Source HgN 13W.  I would say that is a neat design.  Seems to have three connection: one for the filament, one for common connection between filament and cathode, and one for the cathode shield

The cathode shield is in fact floating, like in an FL lamp.  There are 2 contacts for the filament, and one contact for the anode ring at the far end of the bulb.

Thanks, James