Small miniature incandescent light bulb with clear glass dome and threaded brass base. The bulb contains a visible tungsten filament and traces of silvering near the top. Likely used as a signal indicator or pilot light in radio and communications equipment.
This type of bulb was commonly stored in military spare valve and parts kits such as the Case Spare Valves No. 4L (Accession 2020.658). It would have been a standard replaceable component for field radios, transmitters, or Morse signal devices.