The GPO POVT31 is the STC commercial 4101D. The STC version of the Western Electric 101D telephone repeater triode from 1925 has a 4 prefix to denote the UK product. Standard Telephones and Cables separated from the parent American Western Electric. The Australian subsidiary made the same valve but their prefix was SY.
The dual inverted V filaments are oxide coated.
The original 101D was introduced in 1921 and was still in production, in an improved form, until the 1950s.
The base cap has all the identification pressed into the plastic and filled with white paint.
The construction features a single vertical glass rod attached to the pinch stem that all the electrodes are supported by. This construction was vulnerable to shock that would break the rod near the base as exhibit M4205E shows.
The twin ribbon filaments. In the foreground the detail of the coating can be seen. The once smooth coating is not adhering well to the base tape. Note also the construction of the ladder grid where individual wires are spot welded to the main supports.
A closer view of one filament.
This microscope image shows the spacing of the individual components. The sloping bar joins the two sides of the anode.
The balloon envelope is 59 mm in diameter, and excluding the UV base pins is 98 mm tall.
References: Data-sheet & 1047 Type POVT31 was first introduced in 1925.