This tube is a multiple-voltage regulator... a gas shunt regulator tube that provides regulated voltages of 280, 210, 140 and 70 volts DC at 40 ma, thus making it useful for preamp/driver circuit projects or circuits that need multiple regulated bias voltages. Very cool looking in operation, too!
Nice 1960's vintage genuine Marconi British made STV280/40 CV1068 Stabilovolt tubes.