Drift Sight, Tail, RAF Mk IV Kit Ref No 1846/40, 21584 1940 Air Forces of USA It is typically installed externally

It is typically installed externally on the side or beneath of the aircraft fuselage

5 inches wide

a portable precision leveling instrument used for laying artillery weapons in elevation

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

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Drift Sight, Tail, RAF Mk IV Kit Ref No 1846/40, 21584 1940 Air Forces of USA It is typically installed externallyHere is a WWII era Tail Drift Sight, Mark IV Kit, as used in bomber and transport aircraft of the British Royal Air Force with navigator aircrew, such as the Avro Lancaster, and likely used in other bombers such as the Vickers Wellington and Short Stirling. This is a complete set that includes the optical periscope, base plate, two aluminum drift plates that screw onto the base plate, an electric cord for illuminating the sight, and wooden carry case