The 6H6 is the smallest of the metal valves we have seen. RCA introduced the glass foot with metal envelope can in 1935. The construction is of a pressed glass disc holding the electrodes and sealed into the steel envelope. The lead-out wires were then attached to the octal base, also introduced in America in 1935. In subsequent years the glass foot was used to hold hard metal pins and the all glass valve was born. The glass enveloped 6H6 valves used pinch construction and were much larger physically.
In the metal envelope, and sharing a common heater connection are two separate signal diodes.
The data sheets give the design use as am demodulation and AGC rectification. But double diodes found many other uses such as level clamping in TV receivers, and pulse shapers in radar systems.
The General Electric name plus Radiotron usually associated with RCA and finally Licensed by Thermionics Ltd.
The GE logo and the letters RCAF and up arrow. The latter combination indicate a product for the Royal Canadian Air Force. And so it is probable that this valve was originally destined for a WWII airborne radar set.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 28 mm tall.
References: Data-sheet & 1040. Type 6H6 was first introduced in 1935.