The PM1DG is a bi-grid valve and not a screened grid valve or tetrode. With one grid acting as a space charge grid with a small positive voltage applied the anode voltage can be reduced to a very low value. This was the basis of the Unidyne receiver.
Mullard suggest that the PM1DG can be used as a frequency changer or as an audio amplifier. Operating curves can be found in the article above.
The anode is mounted horizontally.
The identification is etched into the dome.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 90 mm tall.
References: Data-sheet, Data & 1043. Type PM1DG was first introduced in 1931. See also1931 adverts.