The VT20 battery triode was designed as an audio output valve. With a maximum output of 190 mW it would be used with headphones and not a loudspeaker. The VT20 became the CV1020 when the Common Valve system was introduced.
Like VT20 this valve is marked as from Cossor but the shape and base look like typical M-OV output of the period.
The fact that this was produced for the Air Ministry is clear from the etched AM logo.
The dome has the Cossor branding etched into it.
Another specimen with a better formed crown between the etched A and M.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
Reference: Datasheet. Type VT20 was first introduced in 1929.