The 6K7G is a screened pentode and was designed for intermediate frequency amplifiers in superhet receivers. It is high impedance type allowing for a high Q in the tuned anode circuit.
See also6K7 metal envelope version and 6K7GT glass tubular type.
The internal screen making external metalising paint unnecessary.
The lower screening disc and the wire grids above.
The classic envelope is 38 mm in diameter and, excluding the IO base pins the valve is 95 mm tall.
References: Data-sheet, 1040 & 1043. Type 6K7G was first introduced in 1936. See also1936 adverts.