The engraving of PATT 8665 on your camera is similar to one, shown at page 38 in Laney's Leica Collector's Guide book, which is [a IIIc] from 1940/41 and probably a captured camera. Laney pondered as to how various Leicas came into the hands of the British military during WWII. It would seem to me that during the war there were two possible avenues (a) in response to requests to civilians to donate their cameras and (b) through capture in and around battlefields. There is no such mystery about your camera which dates from after the end of WWII. As well as PATT, some Royal Navy cameras are marked with AP for 'Admiralty Pattern'. Van Hasbroeck in his book shows a Leica IIIa (from about 1935) with PATT 8665 engraved on it in a different position. This may either be a pre-war purchase or a donated camera.