This MH4 has the suffix /K, to denote the Catkin assembly as opposed to the conventional glass envelope type. As can be seen from the picture, this is a strange valve. The Catkin process was introduced in 1933 by M-OV, the valves were very successful but the cost of manufacture lead this production method to be short lived. The Cooled Anode Transmitting (CAT) valves, that the Catkin process was derived from, remained highly successful.
The top envelope is enamelled copper sealed to glass, no internal structure can be seen. The large wide base holds the pinch and a rubber sealing ring. The metal base tube is 25 mm in diameter. The heater rating at 4 Volts and 1 Amp was the UK standard until the American's introduced the 6.3 Volt heater in the mid 1930s.
The thin metal tube envelope is 12 mm in diameter and, excluding the B5 base pins, is 95 mm tall.
References: Data-sheet, 1005, 1043, 3002 & 1040. Type MH4K was first introduced in 1933. See also 1933 adverts.