The PM5X general purpose and audio output valve is part of the 1927 six volt range from Mullard. The details on the valve emphasize that there is practically no glow from the filament. The PM filament was an oxide coated azide type.
Using the microscope this image of the electrodes was obtained. The filament passes through the grid four times.
Another view the form of the grid winding and the copper grid support wires.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 91 mm tall.
References: 1931 datasheet & 1043. Type PM5X was first introduced in 1927.