The GX432 is a noise diode. That is a source of RF noise used to calibrate equipment. The thin tube would pass diagonally through waveguide and the tube probably has low pressure argon or neon gas within that would be ionised with an applied voltage.
From Wikipedia: Long, thin, hot-cathode gas-discharge glass tubes fitted with a normal bayonet light bulb mount for the filament and an anode top cap, were used for SHF frequencies and diagonal insertion into a waveguide. They were filled with a pure inert gas such as neon because mixtures made the output temperature-dependent. Their burning voltage was under 200 V, but they needed optical priming (pre-ionizing) by a 2-Watt incandescent lamp prior to ignition by an anode voltage spike in the 5 kV range.
The anode loop.
The MES base cap and coiled coil filament. The base tube is 17 mm in diameter.
Another view of the filament.
The thin glass tube envelope is 6 mm in diameter and, including the mes base, is 244 mm tall.