This exhibit is a cold cathode discharge tube. Most likely it is a voltage stabiliser, but when burning the glow changes intensity with current and so it could have been used as an indicator. This possibility occurs as most voltage stabilisers have an anode that encloses the cathode and thus offers better stability of the electrodes.
The U shaped outer electrode with a flat plate central electrode.
The central plate is very well aligned to be parallel with the outer electrode.
The wide glass tube envelope is 32 mm in diameter, and excluding the B4 base pins is 102 mm tall.
Updated March 01, 2013.