Shulachan Aruch (OC 135:10) holds, and Rama (there) seems to also hold, that if a kohen (or levi, m.m.) is called as acharon then the gabai should say "af al pi shehu kohen". The purpose of this is to announce that he's a kohen, so people don't think he's an invalid kohen. Do any Ashk'nazim use this wording, as opposed to saying just "p'loni ben p'loni hakohen"?
In regard to the "af-al-pi shehu kohen", we are Ashkenazim and my husband is a Cohen. Some years ago we spent Shabbat in Venice, Italy. By the time they got around to asking my husband who he was, they had already passed Cohen. So they said, "No problem." They called him up as Chamishi, with the addendum of "af-al-pi..." So here's a case of not even being Acharon. Don't know if Italian minhagim are that different from Sephardic, in general, in this practice.