The MV43 is a neon filled voltage stabiliser tube. No operating details have been located and no reference to any GPO voltage regulators.
Putting it on test revealed that it behaved exactly the same as a 7475 regulator with the neon discharge starting at around 140 Volts and burning at 100 Volts at 4 mA.
The simple construction of two concentric cylinders above the pinch.
The cylinders are not perfectly aligned and this could be damage over the years as there are no mica supports holding the electrodes in place.
The wide glass tube envelope is 26 mm in diameter, and excluding the B4 base pins is 66 mm tall.
