The DER stands for Dull Emitter Receiving. It is a triode and follows early valve design layout the of a horizontal anode cylinder containing a wire helix grid and a single thin thoriated tungsten wire filament.
The valve is marked to indicate that the filament voltage is 1.8 to 2.0 Volts at 0.4 Amps. Clearly designed for use with a lead acid accumulator for the filament supply. The anode voltage is to be 30 - 50 Volts. The DER etched identification is on the lower left and above it is the BBC stamp. This was the first stamp with both BBC and Post Master General identification. This stamp was used from 1st November 1922 until September 1924 thus giving a date range for this exhibit.
Looking up from the base one can see the electrodes. Even the filament can be seen to be intact.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins is 90 mm tall.
Reference: 1003. Type DER was first introduced in 1921.