The QS150/15 is a miniature voltage stabiliser.
The striking voltage is 170 and for burning currents of 2 - 10 mA the regulation is 5 Volts. The electrode at pin 4 is the priming anode and it should draw no more than 500 μA.
The bright anode with no view of the other electrodes.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 47 mm tall.
References: Data-sheet. Type QS150/15 was first introduced in 1954.