The STV 150/200 is a multi section voltage stabiliser. The diagram in Radiomuseum shows the three main connections connected across the mains with a current limiter in series. The centre electrode is grounded and so forming both a positive and negative rail. No specific ratings have been found but looking at other STV devices the gap voltage is probably 150 Volts per gap and the maximum tube current is 200 mA.
Our other STV150/200 has a frosted glass envelope.
The discharge is contained within the long cylinder. The cylinder itself is held rigidly in place by a collar around the pinch stem. The pinch itself has only three wires passing through it. This would tie in with the diagram that showed this cold cathode device as just three electrodes.
The base. If just three connections are passed through the pinch the -C and 0 markings remain a mystery that is yet to be solved.
The top of the discharge cylinder with the outer fingers holding the top of the cylinder tight to the glass envelope.
The dome showing a mica disc and a ceramic formation at the centre. The ceramic looks to be a nut securing the central ceramic rod.
From the pinch to the cylinder the wires are sealed in glass tubes to prevent unwanted discharges.
The wide glass tube envelope is 65 mm in diameter, and excluding the B5 base pins is 262 mm tall.