The 2D2 is a double diode introduced by Mullard in 1937 for battery radios. Indirectly heated battery valves are rare.This exhibit has had the external screening paint scraped away to reveal the inner structure.
The filament is a single insulated strand that passes through the cylindrical cathode. The anodes are just wraps of tape that have been spot welded to the anode support struts.
A closer view of the electrodes.
The balloon envelope is 28 mm in diameter, and excluding the B5 base pins is 62 mm tall.
References: Data-sheet, 4048, 1040 & 1043. Type 2D2 was first introduced in 1937.