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Wikipedia:- "After tests with two experimental sights which had begun in 1939, the first production gyro gunsight was the British Mark I Gyro Sight (left), developed at Farnborough in 1941, and prototypes of which were tested in a Supermarine Spitfire and the turret of a Boulton Paul Defiant in the early part of that year. With the successful conclusion of these tests the sight was put into production by Ferranti, the first limited-production versions being available by the spring of 1941, with the sights being first used operationally against Luftwaffe raids on Britain in July the same year. The sight had a number of drawbacks however, including requiring the pilot/gunner to look through a small aperture, so production was postponed and work started on an improved sight which would incorporate a normal reflector system instead. This new sight became the Mark II Gyro Sight, which was first tested in late 1943 with production examples becoming available later in the same year.

The Mark II was also subsequently produced in the US as the K-14 (USAAF) and Mk18 (Navy)"