The EN31 thyratron is a high frequency type. Mullard describe the EN31 as a thyratron for use in HF timebases and control equipment. The maximum pulse repetition rate is given as 150 kHz. The control ratio is 35 and the voltage drop across the conducting valve is 33 Volts. It is an early type introduced by Mullard and was probably used in radar, TV and test equipment.
An interesting feature is the mesh anode.
Construction details are clearer. The grid has cooling fins attached that can be seen as black squares connected to the copper support rods. The cathode connection and the separate heater leads can be identified. The pinch is on the right.
The heater seems to be formed on a ceramic former that is further insulated before being inserted into the thin cathode tube. Note the ribbon spot welded to the cathode that takes the connection to the lead-out wire.
The classic envelope is 40 mm in diameter and, excluding the IO base pins, is 98 mm tall.
References: Datasheet & 1040. Type EN31 was first introduced in 1947.