The SG215 tetrode was introduced in 1928 with a screw top cap. The 'top hat' top cap appeared in the mid 1930s. It is a screen grid or tetrode valve for RF amplification in receivers.
The formation of AEI saw the Mazda valve brand being transferred from BTH to Ediswan.
The screen grid can be seen through the hole in the silvering left when the getter was fired.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 114 mm tall.
References: Data-sheet & 1043. Type SG215 was first introduced in 1928. See also 1928 adverts.