What makes you say that the Fortress I showed was part of 100 Sqn. - lanc? The Fortress I showed is carrying a 'RU' prefix - which 100 Sqn. never had. Also, 100 Sqn. never had B-17s - the RAF official squadron history states "On 15 December 1942, No 100 Squadron reformed at Waltham with Lancasters and commenced operations over Germany..."
Another site states that 100 Sqn. was part of Bomber Command - 1 Group.
I may be wrong here but the only way I see it possible is if they had the Fortress for an extremely short time or Coastal Command has a different system.
Am I wrong, lanc? Sorry, it was bugging me when I saw the picture and I was trying to find the squadron then I remembered this thread.