The 220DD is an indirectly heated double signal diode with a common cathode. It was designed for battery sets using automatic volume control (AVC) now known as AGC (automatic gain control).
Cossor used paper labels to add the Type designation to their valves. The paper is missing but the dome has makers information etched into it. The filament is intact and runs at 2.0 V at 0.2 A.
The electrode assembly showing the anodes, cathode tube, insulated filament and use of mica's top and bottom.
This image shows how the two separate anode plates are wrapped round the cathode.