The DER was a landmark valve and marks the move away from the bright emitters of the early years. The DER stands for Dull Emitter Receiving and was introduced by M-OV in 1921 as a general purpose valve.
This exhibit carries the Marconi branding. and the moulded Bakelite base has spring pins that the wires connect to outside the base.
Seen through the envelope, the cylindrical anode with helical grid and single strand filament follows the design pattern of the early R type valves. The filament in this case would be of Thoriated Tungsten and operate at an orange/yellow temperature (about 1800 °C). The BBC stamp that is visible to the left of the picture carries the words 'Entirely British Manufacture'.
The central fine filament wire is just visible.
The anode and grid construction is easier to see in this detail picture.
The pinch and supporting wires can be seen.
The balloon envelope is 44 mm in diameter, and excluding the B4 base pins is 77 mm tall.
References: 1004. Type DER was first introduced in 1921.