With IWC Excerpt from the Ledger dated 18 September 2009 confirming the sale of the present watch to S. Heindorf, Berlin, on 24 September 1940.
In 1940, IWC produced 1200 examples of calibre 52 19''' H 6 S.C., numbered 1013801 to 1015000 out of which 1000 examples of this "Die grosse Fliegeruhr" or "Big Pilot's Watch" were made for the German Air Force, following strict specifications in regard to accuracy. The movements were fitted with an indirect sweep centre mechanism with hacking feature, the balance is stopped and hands can be set while the crown is pulled out. The large size of the crown allows winding and setting even when wearing gloves. 1000 of these movements were cased in cases numbered 1033201 to 1034200 and shipped to Siegfried Heindorf in Berlin.
The inside of the case backs were engraved with the following military markings:
B-Uhr (for Beobachtungs-Uhr or Navigation Watch)
Bauart (manufacturer): I.W.C.
Werk Nr. (movement no.):
Anforderz. FL 23833 (classification number)
As of around 1937, watches used by the German Air Force were fitted with such a panel indicating manufacturer, serial number and an "Fl" number. In order to disguise information regarding manufacturer and quantities produced, these indications were coded from 1941 onwards.
The watches were primary tested by the Deutsche Versuchsanstalt für Luftfahrt DVL (German Aviation Research Institute) and classified with a so-called "Fl.-Number". The Fl. stands for flight and the first two digits, 22, for flight control; 23 stands for navigation and 25 for radio surveillance, followed by the three digits serial number given by the DVL.
For a description and illustration of the model supplied by IWC see Military Timepieces by Konrad Knirim, pp. 351 - 352 and Die Fliegeruhren von IWC by Rüdiger Bucher and Christian Pfeiffer-Belli, pp. 73 - 81.