The DET23 was used in airborne radio. The UK services purchased to a specification from both M-OV and Mullard. The M-OV valves had a black label as here and the Mullard a white label. Both should have given the same performance, but in practice the M-OV valves had a slight edge. Over time the specification from the RAF altered to favour the M-OV DET23, which then took the greater part of the business.
Technically the difference in the two makes was a slightly thicker wire gauge used in the M-OV for the flying leads. These were made of red platinum at 0.6 mm diameter. The thicker wire increased the problems of achieving a good durable vacuum seal, but avoided the operational dip in performance over a narrow band of frequency. The latter effect being known as 'suck-out'. The DET23 was in production from the post war period up until the mid 1950s.
This physically small disc seal valve has, of course, co-axial connections. The heater and cathode are opposite the main envelope. The grid has a circular connection to the flat internal metal mesh. The anode appears to be copper and has a flat surface facing the grid. For an example of the construction technique see CV90
A grounded grid configuration would allow for a two cavity mechanical design.
The frame grid in the centre is mounted very close to the flat end of the anode and the flat top cathode. The grid wires are visible as are the locating pins that hold the grid in the copper ring.
From this side the rectangular frame of the grid support can clearly be seen as can the wires from the light reflected of the copper anode. The grid would be wound under great tension and then the wires brazed to the support. With the wires fixed, the wire on the other side can be cut away.
The M-OV black label.
The disc seal envelope is 15 mm in diameter and, including the base pin, is 60 mm tall.