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 superhets was in fact the DH63, a copy of the American 6Q7G, whose characteristics were more suited to the job.
The shape is close to that used by Cossor in their DDT and the two companies certainly had close ties.
The triode has the mesh anode and the diode anodes are the rods within cylindrical shield below the triode.
This image shows the electrodes in close-up. The insulation on the heater wire is clearly seen as is the strap connection to the cathode tube.
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.