44-84864 to NACA in Sept 1945 as a test vehicle for a variety of projects. One was to install the P-51H rudder 'cap' on the P-51D vert stabilizer. At NACA it was identified as NACA 126. It is pretty rare as it is one of the few 'originals' not built up after a major crash
It is a P-51D-25-NT produced in Dallas - long after the various rudder mods were tested, including P-51D-5-NA 44-13253 and 257 and long after the mod was applied on XP-51G prior to P-51H development.
If you wish to model it in original condition, it should have the NACA logo tail band with "NACA126" directly below the tail band. If pre 1947 condition the Star/Bar will be WWII style, post 1947 it will have USAF red line in the 'bar',,, and it will Not be polished aluminum - just NMF on fuselage and silver paint on the wings.