The PM2DX was introduced in 1928 and is described by Mullard as being for detector and general purpose applications within battery broadcast receivers.
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 early production featured a balloon envelope whereas this metallised exhibit is probably from 1932-33 and has a shouldered envelope. Internally the filament coating will have progressed from the messy Azide process of the early PM valves.
Thanks to Tom Miller for sending this image.
The classic envelope is mm in diameter and excluding the B4 base pins, is mm tall.
Reference: Data-sheet. Type PM2DX was first introduced in 1928. See also 1928 adverts.