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.
With the connections reversed, so that the glow forms round the central cylinder, and connected to a modulated current driver set at 16 mA the tube acts as a modulated light source. When positioned within a chamber with a lens at the other end the modulation of speech and music can be recovered at a distance of some tens of feet with ease.
The later G55/1K has the same specification and is a B7G based design.
The cylindrical electrodes mounted on the pinch.
The anode is 22 mm in diameter and the cathode is 8 mm in diameter.
The bright cathode cylinder. At the top can be seen the CV identification showing direct equivalence with the commercial type.
The wide glass tube envelope is 29 mm in diameter, and excluding the B4 base pins is 69 mm tall.
References: Data-sheet. Type G120/1B was first introduced in 1945. See also1945 adverts.