The VX2511 is a hydrogen thyratron. We think it is a triode. The large heat-shielded cathode is inside the lower metal cylinder while the mesh cage is the grid.
Most EEV thyratrons of this class have FX numbers. The specimen looks externally similar to EEV type FX2517. If it is similar in specification, then it would be a triode with a peak power output of half a megawatt and a maximum forward voltage of 20 kV and a peak anode current of 350 Amps. The average current, however, would be limited to 350 mA.
A new search for information in 2013 still revealed nothing specific.
The wide glass tube envelope is 60 mm in diameter and, excluding the base pins, is 184 mm tall.
Updated January 19, 2013.