The Longines Weems 6B/159 models
Numbered in the approximate range 1800/40 to 3700/40. A typical serial number of the watch movement is 5934652 for case 2340/40. This makes the movements a few thousand earlier in production than the smaller A-11 Longines Weems, e.g. with a serial number 5938556 for case 40-662.
The back cover is marked with “Goldsmiths & Silversmiths” who procured the watches. “AM” stands for the Ministry of Air Force (Air Ministry). The case serial number for all five manufacturers of 6B/159 watches runs sequentially, and the model above is 2340/40. Observed serial numbers for the Longines models range from at least 1893/40 to 3691/40.