Question Mark on British Aircraft

Just a guess but could it be for aeroplane #27 in the squadron?
Bill Banaszak, MFE Sr. Who has wondered too

Mad Modeller wrote some stuff in reply to some other stuff...
For what it's worth...
One reason is that the aircraft was a station 'hack', used by anyone who needed an aircraft on short notice, whether to visit the boys at the next station or for combat if their a/c was u/s. Another is that sometimes units on the same airfield shared some of their a/c, so a code letter wasn't always assigned. Yet another reason I've seen given is that, as Bill says, it was No 27 in the sqn - either that, or number 13...
I'm sure there's other reasons too - these are a few I've read about.
RobG (who wasn't there, so makes no claim to sure knowledge)