Die Grosse Fliegeruhr: this model is one of the watches delivered to the Luftwaffe, the German Air Force, who supplied their pilots with a wristwatch that was accurate, robust, and easy to read during missions. A good example of their first use is the Battle of Britain or operation SeaLion (August 1940- May 1941). Only movement numbers between 1.013.801 and 1.015.000 and case numbers between 1.033.201 and 1.034.200 are considered correct for the 1940 Grosse Fliegeruhr. The inside case back is engraved: - B-Uhr: Beobachtungs-Uhr (German for Navigation Watch) - Bauart, (German for Manufacture): IWC - Werk Nr, (German for Movement No.): 1014849 - (Anforderz, German for Classification Number): FL 23883. The FL 23883 number indicates the watch’s assignation. FL stands for flight qualified; the first two numbers indicate the department the watch was assigned to. 22 was the flight control department, 25 was radio surveillance, and 23 was the code number for the navigation department. The following numbers were given by the D.V.L. (Deutsche Versuchsanstalt Luftfahart), the German Aviation Research Institute. Cal. 52: produced in 1,200 examples in 19, (height 6 mm), the caliber was designed in keeping with Observatory watch quality and accuracy. Only 1,000 examples were delivered to the German Air Force. The remaining 200 were used for English deck watches. This caliber is considered to be the largest caliber made by IWC.