Oxygen Regulator Type A-9A
This is a WWII-era aircraft Oxygen Regulator Type A-9A as used across bomber and transport aircraft of the US Army Air Force and US Navy, such as the B-17 Flying Fortress (see image illustrating the location of the co-pilot's regulator from the B-17F flight manual dated 1943). This example has a serial number prefix of AC41, evidence of a year of manufacture of 1941.
The A-9A regulator was a low pressure continuous flow regulator (vs the on-demand, diluter regulator typically used in fighter aircraft) used with oxygen mask A-8B. An A-9A would be installed at each crew position, and may be attached to a portable bottle during movement within the aircraft during flight.
The A-9A regulator had one dial with two gauges, and one adjustment knob. The knob was used to set the flow rate, which was indicated on the top gauge on the indicator. The flow rate was to be set to match the altitude of the aircraft as indicated on the altimeter, up to 35,000 ft. The bottom gauge on the indicator showed the amount of pressure (i.e., oxygen) remaining in the oxygen cylinder.
It is in very good condition given its age. Measures 4.5 inches tall, 2.5 inches wide, and ~4.5 inches deep.