The aircraft is painted in the markings of No 112 Squadron Kittyhawk FX760 / GA-? Which operated in Italy with the Squadron between 3 and 22 June 1944, being flown in that brief period by Squadron Leader Attearne [sic] DFC and F/Lt Reg Wild DFC RAAF
("Attearne" is actually S/Ldr Lance Ahern DSO DFC RAAF.)
Initially I thought it might be a placeholder, but there is at least one photo that shows it operationally with the question mark:
This same plane was also flown by Group Captain Billy Drake DSO DFC & Bar and is famous enough to be available in model form. It might be that it started life as a P and the squadron leader was having some fun, but the contemporary photo implies a measure of forethought. Under what circumstances would an RAF aircraft have a question mark as part of its squadron code?