The POVT181 is the UK General Post Office designation of the valve known usually as the RAF Type VT52 and the commercial Mullard EL32.
The design was a particularly economical audio power valve (heater power = 1.26W) and was used where very large numbers of valves were required (eg. in telephone exchanges), consuming power throughout the night when nothing much was happening but it was essential for the whole system to be instantly ready for an emergency.
Their power output capability was small by domestic radio standards but entirely adequate for a telephone line driver. The POVT181 was used in very large numbers by the Post Office Telephones system.
As a single ended output stage the power is 2.8W, when operated in push pull the pair can deliver 7W at 1.5% distortion.
The envelope is also marked as a Type VT52
The anode is formed from thin sheet to have a cylindrical inner surface and four external flanges for fixing to the supports and for added heat dissipation. The anode is blackened to maximise heat loss by radiation.
The classic envelope is 34 mm in diameter and, excluding the IO base pins, is 91 mm tall.
References: Datasheet & 1043. Type POVT181 was first introduced in 1940.