ac (Ascension Island): This code is a vestige of IANA's decision in 1996 to allow the use of codes reserved in the ISO 3166-1 alpha-2 reserve list for use by the Universal Postal Union.
There are three ccTLDs that have been deleted after the corresponding 2-letter code was withdrawn from ISO 3166-1: cs (for Czechoslovakia), zr (for Zaire) and tp (for East Timor).