The last few post and links got me thinking, or more correctly ...looking.
I went through my collection of FAA microfiche, regarding the S versus 5 info posted on various websites, about this particular mustang.
This info, from FAA fiche 1977 MMS Code monthly 04-1977
North American P-51D
45-11371 N1051S Mustang Aviation Inc. 6165 W. 10th Ave Hialeah FL.
I think I've got too much time on my hands today :?