The 215SG is a screen grid valve or tetrode, this example was made by Cossor using their 'New Process' that was introduced about 1929.
The 215SG 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 screw top connection places the valve in the late 1920's to early 1930s.
The classic envelope is 42 mm in diameter, and excluding the B4 base pins is 110 mm tall.
References: Data-sheet & 1043. Type 215SG was first introduced in 1929. See also 1929 adverts.