During WW II the German Reichsluftfahrtministerium (RLM) classified all "navigation instruments" from Fl. 23000 to Fl 23999. Fl 23235 for example was used for the "Armbandkompass". The Beobachtungsuhren got Fl 23883, on-board clocks got Fl 23885, Fl 23886, Fl 23887 (Bo-UK 1 and Bo UK-2).The German term „Beobachtungsuhr“ goes back to pilot´s watches of WW II. Strictly translated it has to be an „Observer´s watch“ or „Observation watch“ but on most watch related fora you`ll read „navigator´s watch“, „pilot´s watch“, „B-watch“ or even „Flieger“, which is the expression I like most.
These B-watches have been made for the German Luftwaffe by only 5 manufacturers
A. Lange & Söhne
Laco (Lacher & Co)
Stowa (Walter Storz)
Wempe (Chronometerwerke Hamburg)
IWC
and contained high quality pocket watch movements:
Lange & Söhne: cal. 48/1
(due to limited capacities watches have been assembled by Huber/Munich, Felsing/Berlin, Schieron/Stuttgart, Schätzle & Tschudin/Pforzheim, Wempe/Hamburg).
Laco: Durowe cal. D 5
Stowa: Unitas cal. 2812
Wempe: Thommen cal. 31
IWC: cal. 52 SC (SC= „seconde central“)
http://forums.watchuseek.com/f367/what-engraved-numbers-side-case-laco-fliegers-not-crown-side-657607.html
