The STV75/15/II is a small cold cathode voltage stabiliser. The ratings are most probably a maintaining voltage of 75 Volts at currents of up to 15 mA. The striking voltage is around 100 Volts. The connections are via a small bayonet base, exactly the same as would be used for a neon indicator.
The label indicates that this is part of the Marconi Stabilovolt range. See alsoSTV280/80 & VS68.
The bayonet connections and locating pins.
The ceramic base to the anode with the cathode passing centrally into the anode.
The wide glass tube envelope is 28 mm in diameter, and including the bayonet base is 72 mm tall.