The only thing we know for certain is this has a neon filling and strikes at around 200 Volts. The construction is a long thin tube with connections at each end. The negative connection leads to a long central rod, the positive divides into two rods that are held in a mica disc with the central rod between them. The discharge initially is dim and fills part of the tube. With increasing voltage the glow increases in intensity and moves along the tube. Movement of the glow with increasing voltage seems to be close to linear only at the last third of the length.
Noise sources are similar in external appearance but have electrodes at each end and the discharge runs along the tube length. The attached PDF illustrates the noise source construction.
The neon tuning indicator also has a similar construction. The 3184 illustrates the type.
The central rod and end connection.
The discharge end.
The thin glass tube envelope is 12 mm in diameter and, including the base caps, is 213 mm tall.