Cooke, Troughton, & Simms
Drift Sight, Tail, RAF Mk IV Kit Ref No 6B/110, 1939
Here 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 kit includes the optical periscope, a base plate, and the aluminum drift plate that screws onto the base plate, and wooden carry case with leather strap. Stamped on the label on the case of the periscope is Serial No. 2980/39, indicative of a year of manufacture of 1939.
The periscope would be positioned to view the terrain directly below the fuselage while in flight. The angle of flight is then aligned on the sight. As the terrain passes below the aircraft, it travels at an angle that directly indicates the angle of drift of the aircraft caused by cross winds. By rotating the sight to align with the angle of terrain travel vs the intended aircraft course, the angle can be obtained. This angle is then used to calculate course corrections.
The wooden case is experienced but intact. The scope itself appears to be in better than expected shape for an instrument of its kind. The rubber eyepiece has gone by the way, apparently melted as evidenced by the residue in the case and on the instrument, which was attempted to be removed. The knobs turn and the scope rotates in its base when installed as intended. The case measures ~14 inches tall, ~8 inches deep, and ~6 inches wide.