Maintenance Numbers in Flight

I have long been under the impression that once an airframe was allocated an ‘M’ number it was, effectively, out of active service and in use for ground instructional purposes (or similar). The FAA had some airframes, with ‘A’ numbers, which were used for taxying under their own power (at Culdrose) but never flew and I would expect that some RAF airframes would have been similar. In the 1950/60s lots of retired Hunters (with both ‘M’ and ‘A’ numbers) were refurbished for resale after use as instructional airframes. There have even been one or two occasions where aircraft were returned to service after allocation of a maintenance number.
But I have not seen pictures of an aircraft in flight, in service, displaying its maintenance number – other than this one (that I can remember).

[ATTACH=CONFIG]247068[/ATTACH]

Curtiss Cleveland Mk. I of the Royal Air Force flying over Canada
https://en.wikipedia.org/wiki/File:Curtiss_Cleveland.jpg

The Cleveland was the British name for the Curtiss SBC-4 Helldiver for France, five of which were undelivered and so taken up by the RAF. They were transported from Canada to Britain aboard HMS Furious, apparently entering service with 24 Communications Squadron before being used for ground training.
So why does the caption for this Cleveland say it is flying over Canada, with a maintenance number visible on its side, implying that it was not going to be used by 24 Sqn? Unless the caption is wrong, of course, but then it is flying wearing a maintenance number…

Are there other images of similarly marked ‘impossible’ markings?