The AV44 is termed a control diode with a tungsten filament. The design use is for regulated AC supplies. As the diode operates in saturation the anode current is controlled by the filament temperature much more than by the applied anode voltage. If the AC heating voltage alters then the anode change can be used to alter the bias on the grid of the power valve controlling the DC current in a saturable reactor.
The AV44 incorporates a built-in switch which shorts the anode and filament connection should the filament break.
In usual operation the filament would be run at 2.5 Volts and 720 mA. At 100 Volts on the anode the current is 100 μA.
The spring at the top operates the shorting switch in the event of a filament break.
The detail of the construction seen face on.
The construction from the side.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 77 mm tall.
Reference: Datasheet. Type AV44 was first introduced in 1960.