The stv280/40 is a four in one voltage stabiliser that GEC marketed as the Stabilovolt and has a maximum current draw of 40 mA. Also made was the STV280/80 with higher current rating.
It has four anodes, and thus four outputs with gaps of 70 volts each. These output voltages are 70, 140, 210 & 280.
The striking voltage is above 420 Volts and the tube current is 5 - 35 mA. The tube voltage regulation is about a couple of Volts for each 70 Volt step.
The base end showing the insulated leads entering the cylinder.
Inside the dome the outer cylinder is closed at one end and held in place within the glass by the mica sheets.
The wide glass tube envelope is 46 mm in diameter and, excluding the B5 base pins, is 123 mm tall.
References: Data-sheet & R J Sutherland. Type STV280/40 was first introduced in 1936. See also 1936 adverts.