The MHD4 is a double diode triode for detection and first AF amplifier in superhet receivers. It was a cost saving to make all three devices in a single envelope with a common cathode. This style became the normal configuration.
The first MHD4's had screw top caps, this 'top hat' style originated in the mid 1930's.
Eventually the MHD4 with its B7 base was replaced with the DL63 that employed the by then standard octal base.
Although the later Type DL63 had characteristics rather similar to those of the MHD4, the octal successor to the MHD4 in cheap superhet receivers was in fact the DH63, a copy of the American 6Q7G, whose characteristics were more suited to the job.
The Osram paper brand label on this M-OV valve.
The triode at the top of the envelope has a mesh anode. The control grids are wound on copper rods and the cathode tube has a reduced section at the top that is fixed into the top mica.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 114 mm tall.
References: Data-sheet & 1043. Type MHD4 was first introduced in 1933. See also1933 adverts.