The 6K8 triode hexode valve was purpose designed as a radio frequency changer. The valve has a common cathode and introduced the design whereby the triode grid and hexode g1 are one and the same. The triode anode is to one side of the cathode and the rest of the hexode is on the other side.
The 6K8 construction
The local oscillator would produce 7.5 Volts peak to peak signal from the triode section.
The input signal would be coupled to the frequency changer via the top cap connection to grid 3.
The reverse of the steel envelope is plain black paint.
As a metal enveloped valve the top cap connection is interesting. The connector is standard but is set in an insulating disk that is then sealed into a shaped end of the envelope. This appears quite difficult to manufacture.
The IO base.
The thin metal tube envelope is 25 mm in diameter and excluding the IO base pins, is 62 mm tall.
References: Datasheet & 1040. Type 6K8 was first introduced in 1938.