Thank you for the info, Gaucho!
As you correctly pointed out, the registration shown above is for a civilian aircraft.
Confederate Air Force back in the 1960's started placing civil registration numbers under their restored warbirds tailplanes. Following their lead since then, restored warbirds are "de rigueur" repainted depicting a real life counterpart which flew during WWII. The only place where civil registration could be displayed in compliance with the rules, without interferring too much with the rest of the restored livery, is under the tailplane.
However, this doesn't reflect WWII situation, more specifically Tuskegee Airmen's redtailed aircrafts, where military registrations were not moved under the tailplane.
They were not the only ones. The 318th F.S., 325th F.G., 306th F.W., 15th U.S.A.A.F., better known as the "Checkertail Clan", comes readily to my mind. They were another Med theatre famous U.S.A.A.F. squadron flying Mustangs as well, and they overpainted their aircraft registrations entirely with their distinctive, black-yellow, slanted pattern checkers.
Also, Med-operating 15th U.S.A.A.F., 31st F.G. painted red stripes over their Mustang tails, but the registration could still be seen in between the stripes.
Both shown below, courtesy of Wings Palette.
Cheers!
KH :ernae: