The Post Office type POVT25 is electrically equal to the Marconi-Osram type LS5 that was introduced in 1922 but the POVT25 has been fitted with the PO-2 variant of the B4 base. The CV specifications refer to GPO valves as POVT whereas the GPO used the form VT no. xx.
This exhibit is unused and came in its original grey cardboard box. As it was not intended to be sold to the public there was no need to package it in anything but a functional box
The standard LS5 was badged as the POVT25 when used by the British Post Office. The UK government placed telephone communications into the organisation of the Post Office. This lasted until the telecommunications business was sold into private ownership as British Telecom and then BT in the 1980s.
Another view of the valve shows the electrode configuration. It can be seen to be very similar to the LS5A that was introduced in 1924.
The dull emitter filament is a V that is held by a spring at the top to maintain tension when heated. The heater was rated at 4.5 V and 0.8 Amp. The LS5 was designed for audio amplification and had for its day a high anode dissipation of 10 Watts.
The balloon envelope is 52 mm in diameter and, excluding the UV base pins, is 118 mm tall.
References: 4051 & 1004 Type POVT25 was first introduced in 1923.