The Pen220 is of classic Mazda shape and design of the 1930s.
The Pen220 is a battery output pentode for use in the audio output stage of domestic receivers. The Pen220A had a higher anode current and produced 1.1 Watts of audio, again with 7% distortion.
A look at the electrodes shoes the wire grids and the filament wires. The latter form a series of vertical zig-zags and each turning point is supported by a spring.
The balloon envelope is 43 mm in diameter and, excluding the B5 base pins, is 96 mm tall.
References: 1040 & datasheet. Type Pen220 was first introduced in 1931.