P'raps this unusual colour photo gives a clue? Can't remember where I found it but the caption states;
"A Mustang II (USAAF designation P-51A) destined for the RAF being crated and readied for shipment at the NAA plant Inglewood, CA."
(Note the separated wing assembly standing behind)
I dunno what this material being draped over the fuselage is, but it resembles a semi-translucent greyish plastic to me. Plastics were in their infancy about that time however. Note that the worker on the right seems to be drawing the material tight across the cowling. If you blow this image up you can see the fuselage behind the wing is already covered.
I see on the photo of the damaged Mustang above that the aircraft's Bureau number seems to be stencilled on the covering near that pyramid shape just ahead of the where the fin orta be. I wonder if it was stencilled after application of the covering (awkward) or beforehand, because it seems to rule out a 'sprayed' application.
Of course this aircraft is apparently being crated and therefore may have been protected differently to those carried in the open. It does show that markings were already applied though.
Michael