The ECC31 is a medium μ double triode with a common cathode. The CV1285 (formerly NR73) was based on the ECC31. The 6N7G has the same base connections but a much higher anode current rating and was designed for use as a Quiescent Push-Pull output stage.
The two triode unit anodes are parallel to one another.
One anode face view. The control grid is wound on copper supports and is fitted with cooling fins at the top.
The top mica showing the form of the grid cooling fins.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 86 mm tall.
References: Datasheet, 1040 & 1043. Type ECC31 was first introduced in 1942.