The BT79 is a professional valve from AEI and is also marked as a CV372. It is a (triode) hydrogen thyratron for pulse modulator duty. It would work with a peak anode voltage of 3 kV and a pulse power of 60 kW.
It was originally used in some WWII small airborne radars but remained in the lists until the late 1970s.
In-service life was rather short so the valves had to be replaced quite frequently. There are plenty of relatively modern specimens around, mostly in unused condition, but WWII originals are now rare. Modern specimens, like this, include a small hydrogen reservoir cylinder absent in the originals.
The originals have a pinch but this has a pressed glass foot into which the lead-out wires are fixed. The hydrogen reservoir is mounted just above the glass base.
The BT79 and CV372. The small pillow just above the brown base is the hydrogen reservoir.
Through the top screen can be seen the flat disc of the anode.
The base of the single strand filament.
The wide glass tube envelope is 38 mm in diameter and, excluding the UX4 base pins, is 102 mm tall.
Reference: Data-sheet. Type BT79 was first introduced in 1952.