This exhibit came in a box labeled mystery M-OV Valve. Bringing up the heater showed that 6.3 Volts and 0.3 Amps.
The top cap connects to a flat plate held over a bright disc. In the disc is a thin rectangular slot immediately below the centre of the plate connected to the top cap. Below the bright disc is a long thin indirectly heated triode. The top of the small diameter anode cylinder has a slot cut in it. The slot is mounted directly below the slot in the bright disc.
It would appear that varying the grid bias and thus the anode current would also vary the electrons that could pass through the anode slot and be attracted to the plate above the bright disc. What could this be used for?
The first thought was that if the valve has a low pressure gas filling the gas discharge could be modulated by the triode grid - no gas discharge was seen on test.
Another thought could be based on the flat plate being covered with a phosphor. The variable glow would then be reflected from the bright disc. In this second idea the valve would be of the hard vacuum type and the triode section would function correctly. No glow of any kind was seen on test.
If the top plate was connected to a second HT supply the cathode current increased and the top cap curve was slightly more linear than the main triode.
In conclusion: this is mystery.
The pinch and horizontal electrodes. The bright disc is held in place below and via glass insulating beads supports the rectangular plate connected to the top cap.
The small triode with twisted insulated heater leads.
Looking along the triode. The insulated heater, cathode, control grid and anode are all concentric.
The top plate and bright disc.
The slot in the bright disc.
The magnification limits the depth of field in this image but the start of the slot in the triode anode can be seen.
Knowing the heater connections it was a case of trial and error to find the connections with the μtracer. This plot was taken with the top cap left open.
The classic envelope is 44 mm in diameter, and excluding the IO base pins is 99 mm tall.