Oxygen Regulator Assembly, Mark XIA, Ref 6D/751, Spitfire IX, RAF WWII
This is a combination Oxygen Content Gauge and Oxygen Flow Regulator Assembly, British Air Ministry Mark XIA, as used during WWII in aircraft of the Royal Air Force such as the fighter Supermarine Spitfire Mk IX and Spitfire models thereafter. See photo from the Spitfire Pilots Notes showing the location of this assembly.
The pressure gauge in the upper right displayed the contents in the oxygen tank. The oxygen system is opened and flow adjusted by rotating the knob in the lower right. The oxygen flow rate is indicated by the twin floating balls in the flow meter in the upper left, assuming that the flow was adequate when the copper balls were balanced between the two index markers on the left and right of the center of the flow meter. At the bottom left is a switch to immediately raise the flow of oxygen to "High" from Normal in the event of a sudden need.
Barely visible on the data plate is a serial number 3677/ and what appears to be 42 (difficult to confirm), which would be the year of manufacture.
The assembly is definitely 'experienced', and shows the patina of its age. The flowmeter glass is intact and the copper balls are contained therein. The glass on the contents gauge is intact. The flow toggle switch flips up and down as intended. The flowrate knob does not turn. Measures 4 inches square and ~4 inches deep. It has a pink "Repairable" tag affixed to it, but the writing is no longer legible.