I doubt that YT-Z is wearing anything other than the standard Type C1 fuselage roundel. All the RAF Mustang III aircraft that arrived into the UK for the RAF, were stripped and repainted into a standardised C&M scheme before issue to Squadrons. Other required mandatory RAF modifications, such as installation of the Malcolm Hood were also done before issue. The aircraft is directly side on to the camera and looks from the reflection on the Malcolm Hood to have the sun shining reasonably strongly on that side. I've seen a similar effect on other photos of RAF Mustangs (and other types) where the white reflects higher levels of light back to the camera lens, which then gives the impession on film of the white segments of the roundels and fin flash being 'thicker' than they actually are. Having seen some photos where the subject Mustang aircraft was photographed from multiple angles in a short timeframe the same photo session and in some the white segments of the roundels and fin flash are clearly the standard size, but where the light is striking the markings directly the white sections look thicker.