The Cossor DD4 is a low current low voltage indirectly heated double diode. The 4.0 Volt heater was the standard for UK mains valves in the 1930s before the American 6.3 Volts standard was adopted.
Seen from the back the electrode structure is revealed. The heater wires can be seen entering the cathode cylinder. The anode has a centre cylindrical space and the fins help dissipate the heat generated as well as making mounting easier. The pinch can clearly be seen rising from the Bakelite base.
A closer view showing the spot welds holding the anode to the supports.
The balloon envelope is 30 mm in diameter, and excluding the B5 base pins, is 73 mm tall.
References: Datasheet, 1040 & 1043. Type DD4 was first introduced in 1934.