It's a prototype testflight coloring. Each squadron added their own colorings and/or markings on the basic testflight scheme. No bare testflight scheme was used during wwII with P-51D-X model. Smallest addition to the testflight scheme was to paint single elevator stripes and single front wing stripes to wingroot with same color as the engine cowl color used. And of course plane identification markings. I dont recall which squadron used this paintscheme, but I will check if I can find the reference on this one.