This exhibit is a Cossor 215SG screen grid valve that Cossor introduced in 1930 and made with there New Process.
It was designed for RF amplification in radio receivers. The screen provides isolation between the grid and anode circuits and enables greater individual stage gains to be achieved without instability.
The anode is formed as two plates either side of the central screening cage. The filament wire can be seen.
The screen was made at this time as a complete cage.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 109 mm tall.
References: Datasheet & 1040. Type 215SG was first introduced in 1929.