Thank you Brian Abraham, given the history I am unable to provide an explanation.
There were Spitfires at Woodvale while I was there in 1957, but, after all this time, my memory of that photo is vague to say the least.
It is annotated as such in my writing in the album, and I can think of no reason to be mistaken about it.
Also, I was never at any of the units you mention in the aircraft history
As Kitbag points out, an M number was applied to those aircraft allocated for maintainance and training purposes, and to the best of my knowledge, once that letter was applied the aircraft never flew again in service life.