The Duchy of Cromarty was a Manticoran peerage on the planet Manticore.
In 1544 PD, the title was held by Lady Adelaide Summervale, who as the senior member of the House of Lords held the scepter during Queen Elizabeth II's coronation. (MA3)
In the early 20th Century PD, Prime Minister Allen Summervale was the current duke. His successor likely inherited the title after Summervale's assassination in 1915 PD. (HH1)