With the abbreviation FL 23883 where "FL" indicated the word Flieger and "23883" was the specification number of the German Ministry of War, the first two digits indicated the use, so the number 23 was specific for the observation and control of navigation. Instead a number 22 stood for "fluguberwachtung", that is "flight control". Or a number 25 indicated radio control.

The B-Hur watches were worn not only by pilots but often by navigators and technicians present on the bombers to track the routes and target to be hit, so they were used to make the point during air navigation.

In this case the B-Uhr were real chronometers and other types of these chronometers, called RLM and D, could be found on the wrist, strictly over the sleeve of the leather jacket given the large size