This begs the question... Other Mustangs in the group photo with Little Girl in the foreground have those markings (as do other planes in the squadron). Can we not presume they were applied later?
You could presume that, but that would mean that every other marking was painted except the blue lines surrounding the tail number (I've not seen any with the white background in photos), even though the same blue paint was used on the nose, fuselage and rudder. That's pretty thin.
Perhaps the blue lines are too thin to show properly in such a blurry photo? So, at the very least ditch the white background.
Also, the serial looks like it could start with 475 also. really too blurry to tell for sure.