The V914 is an indirectly heated double diode designed for HF signal detection and also for AGC rectification in radio receivers.
The anode seen end on.
The construction is revealed in close-up. The heater and cathode tube is in the centre. The anodes are bright and are to either side of the central cathode.
The cooling fins emerge on either side of the top support mica. The rectangular cathode tube can be seen protuding through the mica in the centre. The etched identification is in reality very faint and revealed by enhancing the image.
The balloon envelope is 42 mm in diameter, and excluding the B5 base pins is 75 mm tall.
References: Datasheet & 3002. Type V914 was first introduced in 1934.