Oxygen Flow Indicator, MK III, British Royal Air Force, 1942
Here is a WWII-era artifact: an Oxygen Flow Indicator, Mk III, Stores Reference 6D/430. Multi-crew RAF aircraft had these installed at crew stations, such as gun turrets, radio operator, navigator, etc., to indicate the gross flow (not the precise rate) of oxygen from the primary oxygen tanks (not portables) to the local junction at each station. Commonly used in aircraft such as the Lancaster, Wellington, Stirling, Mosquito, etc. The serial number on its face is evidence of a year of manufacturing of 1942.
An experienced artifact, to be sure, which has clearly seen service. Measures ~2.5 inches diameter and 2 inches deep. The crown symbol of the Air Ministry is imprinted on its rear case with its stores reference number.