The 220VSG was the first variable-μ valve introduced by Cossor and dates to 1931. The datasheet talks of manual control of the bias point. This was at a time before the widespread use of the superhet in UK domestic receivers and the TRF (Tuned Radio Frequency) or straight set was the norm.
The zinc paint screening is connected to the base pins by the wire that connects to the disc shown. Corrosion can be seen near the top.
Before the introduction of the 'top hat' top cap the screw cap was in use. The zinc paint extends over the base of the plastic cap. The screw top is missing in this exhibit.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 107 mm tall.
References: Datasheet & 1043. Type 220VSG was first introduced in 1931.