This exhibit is a Mullard EF37A in a glass tubular envelope covered in plain grey zinc paint external metallisation. This is the style of manufacture of the CV5080 for the UK government.
The low heater requirement of the EF37A at 0.2 Amps at 6.3 Volts made it attractive for use in the design of the Colossus code breaking processor.
The reverse has the CV Type designation indicating complete equivalence. The CV5080 specification refers to use as an HF pentode with limited microphony and not as an audio pre amplifier valve.
The wide glass tube envelope is 29 mm in diameter and, excluding the IO base pins, is 86 mm tall.
Reference: Data-sheet Type EF37A was first introduced in 1942. See also 1942 adverts.