The VT51 battery audio output pentode was capable of 1.1 Watts at 7% distortion.
This specimen is characteristically Cossor shaped and will be a Cossor Type 220PT rather than a Mazda Type Pen220A. Originally designed as audio output pentodes these economical valves found numerous uses in portable battery-powered military transmitters.
The bright anode and pinch components.
The The grids are clearly visible within the cylindrical anode. The filament was designed for accumulator use, and the maximum anode voltage of 150 Volts matched the maximum HT batteries available.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 94 mm tall.
References: Datasheet & 1043. Type VT51 was first introduced in 1933.