STC is the UK subsidiary of the American ITT company. The odd thing is that this ITT device is marked as Made in England - so clearly an STC product but normally this would be marked as STC as per G240/2D.
The G240/2D is a cold cathode Thyratron. When conducting the voltage drop across the valve is 90 Volts. The discharge is initiated by a voltage on the trigger electrode.
The reverse of the envelope is marked as CV2174.
The main electrode is sheathed in glass.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins, is 72 mm tall.
Reference: Datasheet. Type G240/2D was first introduced in 1949.