A-15 Oxygen regulator.
Portable Warbird Oxygen Diluter Demand Regulator. Manufactured by Scott. Type A-15. It has the 53 part number prefix but oddly the data plate denotes it Property AF US Army. It has a clip on it that attaches to aircrew uniform when used, and the instrument and bezel are still attached with a Perspex glazing unit.
Part number is AD1876. Although a post war unit it’s the same as the wartime issue unit. I actually think it was issued in 1953 but is probably a WWII item as the label states ‘Property AF US Army’.
Last photo shows the item attached to a walk about Oxygen bottle
A nice complete item