This mystery came in a box labelled as a Special Diode. It looks to be an EHT rectifier with a Paxolin base and side contacts for the filament.
The Paxolin base is threaded. The possibility is that a handle was fitted as with WWII German valves like the RV2P800. This would make it field replaceable. The design looks to operate at a few kV as would be used for a CRT display.
The anode bell is supported at the top and the two supports from the pinch end in a single strand filament. The valve is not gettered and the filament is probably tungsten. On test it glows brightly at 1.2 Volts and 600 mA.
The side contacts are tapered at the end. Further evidence of a plug-in design.
The microscope shows a single wire filament. It is spot welded to the supports but looks as if the supports melted and not the wire. Another indication of a tungsten filament.
The thin glass tube envelope is 14 mm in diameter and, excluding the base pins, is 72 mm tall.
Updated March 12, 2021.