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 vertical supports for the top mica also act as conductors. The left support connects one anode to the pinch. The right support is connected to both cathodes by thin tape and thence to the pinch.
The twin triodes with the central vertical rods holding the top mica.
The side mica sheets at the top fit tightly into the glass and hold the electrodes rigid. The control grid is wound on copper rods with heat radiating fins at the top. The cathode tube is shaped to a small diameter for securing in the top mica.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 88 mm tall.
References: Data-sheet, 1040 & 1043. Type ECC31 was first introduced in 1942. See also1942 adverts.