The G120/1B is a neon voltage stabiliser with open cylindrical electrodes. The central cylinder is the cathode and the outer cylinder is the anode and the orange glow forms round the anode.
The regulation of the voltage across the tube for variations of tube current between 2 mA to 30 mA is just three Volts.
The BBC Valve Section label is dated 15 June 1955.
The reverse carries the CV438 Type designation.
The construction of concentric cylinders.
The wide glass tube envelope is 29 mm in diameter, and excluding the B4 base pins is 72 mm tall.
References: Data-sheet. Type G120/1B was first introduced in 1945. See also1945 adverts.