The A2/DC/S is not a valve, it is a concentrated arc lamp. These point light sources operate at around 20 Volts DC and about 25 mA when burning but need a pulse of about 1,000 Volts to ignite.
The electrodes are a disc with a central hole and a rod that extends to about a milimetre below the hole in the disc.
This arc lamp featured in an article for a light transmitter in the Radio Constructor in the 1960's where it was the anode load of an audio output valve. Striking was by means of a pair of spark gaps and a car ignition coil.
The disc electrode with rod below.
The three base pins.
The geometry of the lamp.
The thin glass tube envelope is 14 mm in diameter, and excluding the base pins is 40 mm tall.
Reference: Radio Constructor.
Updated April 17, 2013.