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 between 133 and 147 Volts per gap and the maximum tube current is 200 mA. Striking occurs above 190 Volts.
The Wehrmacht was the unified armed forces of Nazi Germany from 1935 to 1945 and STV were a Berlin company. From about 1940 their products were branded Telefunken. The BAL code denotes the place where the valve was tested and accepted. BAL codes suggest Luftwaffe and RLM Eigentum translates as property of the Air Ministry.