Fuel Quantity Indicator, Hydrostatic, Japanese Naval Aviation

This is a fine example of the WWII-era hydrostatic Fuel Quantity Indicator, manufactured by the Japanese under license from the US manufacturer Pioneer. This particular model was used in the A6M2 version of the Japanese Navy Figher Zero, aka "Zeke" (according to the Funatsu Aviation Instruments Museum Website.

Hydrostatic fuel quantity indicators are readily identifiable by the pump knob at their base. The indicator essentially is measuring differential pressure in the fuel line/tank. Once installed, the gauge always shows the amount of fuel, subject to errors caused by changes in air pressure.To obtain the current exact and correct fuel reading, the pilot pulls out the pump knob to its limit and releases it.  (per Robert Mikesh's book Japanese Aircraft Equipment 1940-1945).  The display indicates fuel quantity on a scale of pressure.

It is in very good condition given its age. The glass is intact. Measures ~3.25 inches across and ~ 3.5 inches deep.