The 0A3 gas filled voltage stabiliser would strike at 100 - 105 volts and then burn at 75 volts. For tube currents of 5 - 40 mA the voltage would only change by 6.5 volts and over the range 3 - 30 mA the regulation is 3 Volts.
The construction consists of a bright outer cylinder and an inner conductor. The discharge takes place within the cylinder.
The mica obscures the inner construction but at the top a wire can be seen protruding from the outer cylinder to close to the central rod. This small gap will facilitate the striking of the discharge.
The classic envelope is 37 mm in diameter and, excluding the IO base pins, is 89 mm tall.