I notice the lack of hyphens. I thought that may be the explanation.

Incidentally, haven't the Scott and Buccleuch titles passed through the female line, Ann(e?) Scott being the sole heiress and being married off to James, son of Charles II and Lucy Walters, who adopted the name of James Scott?

Whittakers has a list of family names of peers with the relevant title, and the Duke appears under M for MDS, not S.