The GPDE probably stands for General Purpose Dull Emitter and with a filament rating of 1.5 to 2.0 Volts at 0.15 Amps, the filament is probably thoriated tungsten.
The envelope carries a paper label with the BBC stamp. This type two stamp was used between September 1924 and 1927 and thus we have an approximate date for the valve. The pins have the lead-out wires attached at their top and are of simple split-pin construction.
The silvering of the envelope obscures the electrodes but at one angle the anode etc is visible. The anode can be seen to be a flattened cylinder mounted vertically. The grid follows the shape of the anode and is fixed to a single support rod. The filament appears to be single hairpin inverted V.
The wide glass tube envelope is 28 mm in diameter, and excluding the B4 base pins is 93 mm tall.