The 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 1930's before the American 6.3 Volts standard took over.
The type identification is printed onto the glass. The anode looks woven but is in fact sheet metal that has holes punched into it.
The two anodes have hollow cylinders where the support for the opposite anode passes to the top mica. The white insulation on the two hairpin heater strands is in the centre of the cathode tube. The heater does not make a complete contact with the cathode and so most of the heating is by radiation. The pink circles on the glass look to be a quality control stamp.
A close-up of the structure showing the central coated cathode. This example of the DD4 is 10 mm taller than the Cossor version of the DD4.
The Balloon envelope is 30 mm in diameter, and excluding the B5 base pins, is 81 mm tall.
References: Datasheet, 1043 & 3002. Type DD4 was first introduced in 1934.