The Pen40X is not mentioned in our source material. It is possible that the identification is incorrect. The photo shows what may be an M-OV Type HL410 made in the late 1930s by which time it was already obsolete. We think that this has been relabelled and supplied as a Mullard PM4DX (NR15A).
Note the different appearances of the azide (PM4DX) and oxide (above) versions of what we believe to be the same nominal type.
The three filament tension hooks can be seen in the picture, this gives six strands of emissive wire within the operating region of the valve.
The balloon envelope is 44 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
References: Private communication.
Updated December03, 2012.