This exhibit is a Mullard 2D21, the final digit is missing but the rectangular box anode is characteristic.
The 2D21 thyratron is, as usual, a gas filled valve, in this case the gas at low pressure is xenon. This example is of tetrode construction and was designed for relay or grid-controlled rectifier applications.
When a certain critical anode current flows the ionisation of the gas begins an avalanche discharge that overrides the grid control and leads to saturation current flow. The anode current can then only be extinguished by reducing the anode volts below the ionisation threshold.
The gm value in a normal hard valve is replaced by the control ratio parameter. In this valve the ratio is 250. At saturation the valve voltage drop is 8 Volts.
The anode is shielded down to the pin.
The heater is a hairpin of coiled and insulated wire.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
References: Data-sheet & 1040. Type 2D21 was first introduced in 1944.