The PM2DX is described by Mullard as being for detector and general purpose applications within battery broadcast receivers. It was introduced in 1928.
The envelope is smaller than the PM2 and the electrodes are obscured due to the tape that has been added to keep base attached.
The filament is for accumulator use and the anode is designed for dry battery operation with a maximum anode voltage of 150 Volts.
The PM2DX was also popular as a driver valve for quiescent push-pull output stages such as the PM2B.
The balloon envelope is 40 mm in diameter and excluding the B4 base pins, is 78 mm tall.
Reference: Data-sheet. Type PM2DX was first introduced in 1928. See also 1928 adverts.