I've added a couple more to the above list.
Here are some examples that are known to have been assigned to various units during WWII:
44-12125 - a P-51K that served in the 78th FG, based at Duxford, now on static display in the Netherlands, having entered the Netherlands AF following WWII (I think this would make a great "Then and Now")
44-63350 - crashed in 2007, currently under restoration, was painted as Lou IV - was assigned to the 15th AF
"Grim Reaper" - finished in 9th AF markings, was assigned to the 8th AF
"Daddy's Girl" - was assigned to the 8th AF
"Angles Playmate" - finished in 9th AF/Bruce Carr markings, *believed to be a 357th FG combat vet
"Tempus Fugit" - was assigned to the 8th AF
44-63871 - on display in Paris, was assigned to the 8th AF
44-63992 - on display in Sweden, was assigned to the 8th AF
"Jumpin Jacques" - believed to have been assigned to the 332nd FG (Tuskegee Airmen)
"Sweet Revenge" - was assigned to the 8th AF
"Baby Duck" - was assigned to the 8th AF
44-72123 - this is one of the original wartime "Millie G's" of Maj. Ed Giller, assigned to the 55th FG, currently mounted to a pole on outdoor display in the Dominican Republic (I really wish this aircraft could be rescued somehow)
44-72395 - currently under ground-up restoration, an original combat vet of the 479th FG, 435th FS of the 8th AF
"Hell-Er Bust" - was assigned to the 8th AF
"Ridge Runner III" - an original combat vet of the 354th FG, 356th FS of the 9th AF, coded AJ-R
44-72777 - believed to have been assigned to the 52nd FG, 5th FS of the 15th AF
"Scat VII" - some believe this is the original aicraft, as painted, but likely only the identity and paperwork can claim it as such (having been built from the ground-up a few different times)
"Petie 2nd/44-72942" - was assigned to the 12th AF
Most of those that are later than this, if they did get shipped out from the U.S., sat at a depot for a while before being shipped back to the States, arriving to Europe too late to see any action.