The Hivac P220 triode is a low wattage audio output valve for battery radio receivers. The load impedance is 10,000 Ohms and the power output is given as 180 mW.
Seen side on the envelope ia largely empty as the front to back dimension is only 5mm. Note the twisted cut in the base pins.
The dome of the envelope carries the identification etched into the glass as well as Made in England. Note the H stamped into the anode. Extra rigidity and a brand reference together.
The mica wings touch the inside of the envelope shoulder. The cross mica holds the two filament tension springs. The filament is oxide coated. The grid wires look un-supported around the supports but are in fact held at each turn by a small closed over notch.
The classic envelope is 39 mm in diameter, and excluding the B4 base pins is 92 mm tall.
References: Datasheet & 1043. Type P220 was first introduced in 1929.