The CVC21 is the only identification on this exhibit. It may be that part of the Type designation has been lost and that is why no details could be found. It looks to be a tetrode thyratron.
The metal evacuation tube and two wire connections. The other connections are to the holes in the metal rings that are separated by the ceramic rings. This is probably both a high voltage and high current device. The use of ceramic permits a higher working temperature than glass.
This is probably the anode connection in the centre.
The planar envelope is 45 mm in diameter, and excluding the base pins is 83 mm tall.
Updated May 26, 2019. shape:planar type:thyratron base: pin: