The CV413 is a cold cathode gas filled relay or thyratron. The prototype was the identical STC G150/2D.
The reverse of the envelope carries the STC logo and the commercial Type designation of G150/2D.
The dome shaped electrode is in fact two concentric electrodes with a gap between.
The cathode is the dark outer electrode. The control electrode is the inner bright ring. The main anode is the central spike within the glass tube. When operating a bright orange (neon) glow covers the cathode.
The control electrode operating at 13 mA.
The concentric electrodes. The central anode is not connected. The blue box is part of my μTracer equipment. The thyratron is connected to the HT unit via my current modulator that is used for demonstrations of optical communications.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 63 mm tall.
