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 Mark I sight had a number of drawbacks, however, including a limited field of view, erratic behaviour of the reticle, and requiring the pilot/gunner to put their eye up against an eyepiece during violent manoeuvres. Mark II Production of the Mark I was postponed and work started on an improved sight. Changes involved incorporating the gyro adjusted reticle into a more standard reflector sight system, a non magnifying optical sight that had been in use since 1918. Reflector sights consisting of a 45 degree angle glass beam splitter that sat in front of the pilot and projected an illuminated image of an aiming reticle that appeared to sit out in front of the pilot's field of view at infinity and was perfectly aligned with the plane's guns ("boresighted" with the guns). The sight sat some distance away from the pilot, so it was safer to use and didn't impair the pilot’s field of view. The optical nature of the reflector sight meant it was possible to feed other information into field of view. In the reflector sight version, range was measured by comparing the wingspan of the target seen through the sight to a pre-set number. The pre-set number was selected via a large dial on the front of the sight, and the range was then measured by turning another dial on the aircraft's throttle. 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. Ferranti built a new factory in the Crewe Toll area of Edinburgh, Scotland to build the sights. This factory would later go on to be the center for Ferranti's long history in radar development. The Mark II was also subsequently produced in the US by Sperry as the K-14 (USAAF) and Mk18 (Navy). The K-14 included two projector systems for the reflector sight, one with gyro correction for attacking aircraft, and a second for attacking ground targets. It was otherwise similar to the British models, although the dial for adjusting the target size was moved to the left side of the sight