Summary

This valve is probably an output triode. Manufacturer: Mullard Radio Valve Co. Ltd. England.
No reference could be found to a PM4D; there is a PM4DX which is a general purpose triode. The PM4D may be a derivative of the PM4 and hence an output valve.
4 volt directly heated cathode. Dull emitter. Anode voltage 125

Physical Description

Conical bulb with 4 pin UX style base. Paper label on base is disintegrating,

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