The MHD4 is a double diode triode for detection and first AF amplifier in superhet receivers. It was an cost saving to make all three devices in a single envelope with a common cathode. This style became the normal configuration and the double diode triode is the last in the Magic Three line-up.
Eventually the MHD4 with its B7 base was replaced with the DL63 that employed the by then standard octal base.
The other side of the metalised envelope carries the printed makers identification.
The base carries more information. Also the final form of the split pin design can be clearly seen.
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 classic envelope is 42 mm in diameter and, excluding the B7 base pins, is 103 mm tall.
References: Datasheet & 1043. Type MHD4 was first introduced in 1933.