The CV1731 (POSV2) is a gas filled (neon) voltage stabiliser used by the GPO. The striking voltage is 160 V and the burning current is from 6 to 75 mA. Within the current range the regulation is 5 volts.
We have a cossor made CV1731 and the construction is quite different.
The inner electrode wires are surrounded by ceramic insulators down to the pinch.
The top of the outer cylinder is open and inside can be seen the flat mesh inner electrode. The outer cylinder is secured to the top mica that is itself a tight fit into the shoulder of the envelope.
The classic envelope is 50 mm in diameter and, excluding the B4 base pins, is 106 mm tall.