The Cossor New Process 220HPT is a battery audio output pentode.
The top view shows the wire grids within the bright anode cavity.
The load resistance is 10,000 ohms and this is greater than high impedance headphones, so clearly this valve would have been used with an output transformer to transform the impedance to that required by a loudspeaker.
The balloon envelope is 43 mm in diameter and, excluding the B5 base pins, is 90 mm tall.
References: Datasheet & 1043. Type 220HPT was first introduced in 1932.