The MH4 is an indirectly heated amplifier triode. The MH4 was also produced in Catkin form in 1933/4. The glass envelope version has a totally different electrode construction but gave the same results. The Catkins were excellent valves but were more expensive to make and were phased out after a few years.
Our other Marconi baloon envelope MH4 has external metalising.
Through the blackened glass one can make out the mesh anode, grid and indirectly heated cathode.
The Marconi paper label.
The dome of the glass has the identification etched into it but no brand name. This way M-OV production was branded just by adding the paper label - Marconi or Osram.
A closer view.
The balloon envelope is 50 mm in diameter and, excluding the B5 base pins, is 113 mm tall.
References: Datasheet, 1005, 1043, 3002 & 1040. Type MH4 was first introduced in 1929.