Hi :-)
The problem is in fact that there is lack of a source for such a shadow the more so that it is clearly seen at that pic that behind small fin fillet fuselage surface runs in typical manner for the whole Mustang family.
No source for such a shadow because early NA-83 and NA-91 are finished in this place totally other, much deeper down to fuselage, than at that pic, exactly as can be seen below at the manufacturing line of both NA-83 and NA-91.
http://img223.imageshack.us/img223/7338/1pj0.jpg