The QS75/20 is a voltage stabiliser tube. The striking voltage is 110 Volts and it will maintain a voltage of 75 Volts plus or minus 5 Volts between 2 and 20 mA.
Keith Watt reports that the GEC CV284's that he has have the separate ignition electrode. The GEC/Marconi datasheet says that these stabilisers do not have the ignition electrode. To be safe connect pin 4 as if it is a priming anode. Connect to the other anode pins via a 15 Ω resistor.
The outer bright cylinder is the cathode.
The central rod is the anode.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 45 mm tall.
References: Datasheet. Type QS75/20 was first introduced in 1954.