Boost Pressure Indicator, MK IIIm, 6A/1581, British Royal Air Force, 1943 C-121 shows a (+) positive reading

shows a (+) positive reading indicating the current is flowing to the battery (charge)

Gun Director Mk 34 was used in USN cruiser classes Baltimore

and fighters P-38 Lightning and P-61 Black Widow

The knob itself is missing

Boost Pressure Indicator, MK IIIm, 6A/1581, British Royal Air Force, 1943 C-121 shows a (+) positive readingHere is a WW2 era Boost Pressure Gauge, 7 to 25 PSI, used to indicate the supercharger power being delivered to the engine when demanded. Used in Merlin powered RAF, RAAF, and RCAF aircraft. According to the RAAF 346 Types and Locations of Instruments in RAAF dated 1945, the 6A 1581 appeared in the DeHavilland Mosquito. This is New Old Stock in its original box dated 1943. Mint condition gauge.