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.
MH4 is an early glass enveloped version with the older style balloon envelope, this exhibit shows the progression to the shouldered envelope and mica supports that are an interference fit into the top of the envelope and thus hold the electrode assembly rigid.
The Marconi name was printed onto the plain zinc paint.
The classic envelope is 45 mm in diameter and, excluding the B5 base pins, is 93 mm tall.
References: Data-sheet, 1005, 1043, 3002 & 1040. Type MH4 was first introduced in 1929. See also 1929 adverts.