As GR2 says, M' is also a possibility.
The M' spelling is particularly useful to be aware of when searching 19th century newspaper sites. Quite often M'Duff will bring up results which don't appear if you enter MacDuff or McDuff.
My 19th century Scottish-born Mc ancestor got a few mention in local newspapers, usually reports of court cases.
There are several examples of his signature. He was consistent, always signing with a large M followed by a tiny c, the form my family still use. His wife couldn't write. Births and deaths of their children were variously registered as "Mac___" or "Mc___" and spelling of the second part of the name wasn't consistent in those records. His name was spelled differently on a census return when he was listed as a lodger. From that record I surmised that he pronounced it differently ("the Scottish way") to how he wrote it.
Local English people pronounced my family's written surname differently again, with emphasis on the "Mac" part. My ancestor's widow's 2nd husband didn't write either. First census after their marriage has her son from her first marriage as "Mack ___". His name was incorrectly transcribed in an index under the second part of the surname with "Mack" as a middle name. It reverts to his father's spelling on the next census and all subsequent documents for him and other members of the family. He and a younger sister could write and he became an insurance agent later in adult life so he was accustomed to keeping accurate records.
Mac/Mc, like the O' prefix, may have been omitted on some records or transcriptions or indexes.
