This exhibit is, we think, a Cossor 215SG screen grid valve that Cossor introduced in 1930. 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 only indication as to Type designation is the lettering scratched on the base. The photographs give clues to the origin.
The anode has ridges to prevent distorting when hot.
The screen is a close wound cage with closing caps top and bottom. This is the earliest style of screen grid, later it was found that a simple helical grid had the same effect.
Seen in close-up the filament can be seen. The shape of the filament looks like the Cossor construction and the base and pins match the 210SPT of 1934.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 110 mm tall.
References: Datasheet & 1040. Type 215SG was first introduced in 1929.