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.
Eventually the MHD4 with its B7 base was replaced with the DL63 that employed the by then standard international octal base.
This MHD4 is an early example of the type and has a clear bulb rather than a carbonised envelope of external metalisation. The top cap is the early 1930s screw type and the other examples that we have use the metal cap.
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.
We also have the MH40 anti-microphany audio triode. This has ceramic top and bottom electrode supports for greater rigidity.
The paper Osram logo and a long stem pinch. The support rods that connect to the control grid appear to run through the metal bell below and on to the pinch. In fact the control grid rods are cut off and do not make contact with the bell.
The bell is one of the diode anodes, the second diode uses the lower rods as its anode.
The mesh anode and within the control grid.
The anode is much wider than its front to back dimension.
The classic envelope is 40 mm in diameter and, excluding the B7 base pins, is 100 mm tall.
References: Data-sheet & 1043. Type MHD4 was first introduced in 1933.