San Martino

The Duchy of San Martino was within that part of the world Sarah Jane Smith recognised as Italy. She and the Fourth Doctor went there in 1492. After the death of his father, Giuliano became Duke of San Martino, surviving a power grab by the next person in the — his uncle, Count Federico. (TV: The Masque of Mandragora)

Behind the scenes
San Martino is a, but it does not name any actual, historic duchy of Italy.