The 0A2 or OA2 is a cold cathode voltage stabiliser. The mean voltage stabilised is 150 Volts. Between the 5 mA minimum current and 30 mA maximum current the voltage regulation is 6 Volts. The striking voltage is 185 Volts.
The bright cylindrical cathode fills the space between the mica supports and the inner electrode is obscured.
The base showing the multiple connections and the central anode.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins, is 59 mm tall.
References: Data-sheet & 1040. Type 0A2 was first introduced in 1954.