The present watch is from an exceedingly small series of observation watches ordered for a special unit of Germany's armed forces, fitted with larger cases, calibre 48 movement and with a different dial layout than the standard issues (see lot 399 in this auction). For photographic purposes the seconds scale was mirror-inverted. Following the first prototypes cased in silver, Lange was to supply 10 examples with nickel-brass cases each month. It is thought that only an exceedingly small number of these prototypes in silver cases were made, rendering this watch a truly rare find.
The models are explained and illustrated in A. Lange & Söhne - eine Uhrmacher-Dynastie aus Dresden by Reinhard Meis, pp. 220 & 221 and Military Timepieces by Konrad Knirim, p. 452.