Ottawa & Toronto Instruments Ltd
Drift Sight, Tail, RAF Mk IV Kit Ref No 1846/40, 21584 1940
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 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 with leather strap. The original lamp is installed but unlikely to work. The remnants of a replacement lamp are stored in its recessed position in the case. Made in England by Ottawa & Toronto Instruments Ltd. Stamped on the side of the periscope is Ref No. 1846/40, indicative of a year of manufacture of 1940.
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, with the rubber eyepiece somewhat deteriorated and a crack in the sight glass at its bottom. 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. A photocopy of the manual is included.