There's several photos of V1 in it's hangar when discovered by the Allies. The initial discovery photos do not show the lettering.
Not sure why it was lettered, as the other P.1101 photos during it's various testing stages don't have the lettering.
I suspect it was put there by a GI, as you can see it was extremely popular with souvanier hunters and various photos show it's condition deteriorating rapidly before it was taken away for evaluation.