Drift Meter, Type B-5
This aircraft Drift Meter, type B-5, was used in several US Army Air Force bomber and transport aircraft, including the B-17 and B-24. The B-5 drift meter is used by the navigator to determine the angle of deviation (drift) between the heading of the aircraft and its actual path over the ground due to cross winds, thus used to correct the course of the aircraft. The instrument's viewing port projects from the side of the aircraft. When viewing the ground through the eyepiece while in flight, the navigator observes the travel of fixed objects across the lens. With a pencil mounted in the receptacle on the movable arm, these objects are tracked across the lens and their path transferred in pencil to the recording pad manually by the navigator. Once done, the drift angle is determined and course correction calculated and communicated to the pilot.
With a serial number prefix of 42, it was made in 1942. It is in very good condition given its age. The drift scale and airspeed/altitude calculator rotate as they should. The pencil arm extends as it should. The viewfinder is clear, however, the mirror has deteriorated but can still be viewed through it, see photo.
Here's a helpful video by another enthusiast on how it works B-5 Drift Meter.