The 2V/400A is a mercury vapour rectifier employing the original American standard AC heater voltage of 2.5 Volts. This exhibit was made by STC in England and STC was the subsidiary of the American ITT company.
The mercury coats the inside of the envelope but a clear view of the electrodes can be seen in 2V/400A.
The underside of the filament section can be seen complete with ceramic insulator. The locating pin for the bayonet holder is on the side of the base cap.
A closer view.
The balloon envelope is 62 mm in diameter, and excluding the UX4 base pins is 156 mm tall.
Reference: Datasheet. Type 2V/400A was first introduced in 1952.