This exhibit is a UK Post Office design known as the POVT148 or VT no. 48. It is an amplifier tetrode with a μ of 4.6 mA/V and an unusual heater rating of 3.3 Volts.
In 1941 the GPO, Army, Navy and RAF Type designations were harmonised into the Common Valve series for all UK Government valve specifications. The POVT148 became the CV1698.
The lettering on the reverse is 36P148.
The cathode is a rectangular tube, the heater looks to be a single hairpin and the control grid has a close pitch and copper supports.
The top shows the pair of wire grids and the flat cathode face with white oxide coating.
The classic envelope is 41 mm in diameter, and excluding the B7 base pins is 103 mm tall.
References: Data-sheet. Type POVT148 was first introduced in 1938. See also1938 adverts.
