Yes it's Saint Helena, Ascension and Tristan da Cunha top level domain. Interesting fact: If it's a two-letter top-level domain it belongs to a country. Generic top-level domains have 3 or more characters.
You've contradicted yourself, .AC is an overseas British possession, not its own country. There are a number of geographic locations that have two letter country codes (.io, etc) which are not members of the UN and are possessions of some other, much more powerful nation, usually as a result of the last 400 years of colonial activity.