Fuel Pressure Gauge, 50PSI, R88-I-1872, Bureau of Aeronautics US Navy P-3 Orion is a pressure gauge which

is a pressure gauge which accepts oxygen in liquid form at the inlet and oxygen in gas form exits at the outlet

See photo of the location of the instrument on the Flight Engineers upper instrument panel from the C-121 flight manual dated 1958

This is a Flap Position Indicator as used in the US Army Air Force Flying Fortress B-17F & G models

that is in a left turn the lubber line moves to the left round the dial

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Fuel Pressure Gauge, 50PSI, R88-I-1872, Bureau of Aeronautics US Navy P-3 Orion is a pressure gauge whichHere is an aircraft Fuel Pressure Gauge, 0 50PSI, made by Bendix, with BU AERO US NAVY on its data label. Measures 2. 25in diameter and ~3in deep. Appears to be New Old Stock.