Or is it even considered one? Once upon a time, it wasn’t unusual for a name beginning with “Mc” to have the “c” raised like a superscript (almost), and a small mark underneath it. Fer example.
Looks like a tilde (~) but I can’t tell from the picture.
I don’t think it’s a diacritical mark (it’s still pronounced McMaster, right), I think it’s just a stylistic choice in the way the word was written. IOW, it’s makes the guys name look nice.
It’s a macron, and indicates an abbreviation. In this case, it represents the “a” left out of “Mac”, and has migrated below the “c”.
Its purpose is to show that the C is superscript, which means that it is meant to be read with a preceeding A (i.e. **M[sup]c[/sup] **is to be read as Mac).
A good explanation here: note the dot, which may be the origin of your bar. (In which case it’s called the “punctum delens.”) From that page:
“When consonants are written in this superscript position it means that they are to be read as being preceded by an a. Thus the medieval scribes wrote gac(h) ‘every’ as a g with a c above it. Úa h-Úmhair modifies this particular medieval contraction by writing the c after the g; but it is still raised in the air by a dot which is placed under it (and there is a dot over it as well to indicate the following h). Readers with Scottish surnames beginning in Mac- or Mc- may spot something significant here. The ancient contraction for mac ‘son’ shows the same evolution. It starts off life as an m with a c written above it, but in time the c is written after the m, though still in a raised position.”
(If I can get the unicode right, gach becomes g[sup]̇[/sup])
Edit: it looks like I did not get the unicode right. :smack:
Well that’s interesting. I was familiar with the macron, but never considered that that usage was also considered a macron. Thanks.
Some interesting discussion here: http://typophile.com/node/39917
I can’t find anything about it in my copy of Bringhurst’s The Elements of Typographic Style.
Interesting. I bear such a surname and was taught to put two short vertical lines under the c when I learned to write my name in the early 1960s. A couple of years ago I was reflecting with my father on how that fashion seems to have disappeared. I’m not sure what to blame; typewriters and moveable type were already common, even in my childhood. 
Great thread, close to my heart. Everyone into this should read Pause and Effect: Punctuation in the West by M.B. Parkes. (Note how I skirted the damn genitive apostrophe? Should name initials be close up? [Should “close” be pronounced with a “z” sound, like some defective verb? [[Yes, for copy editors and typographers.]])
The history of the comma, it turns out, is critical to the inventions of music notation in the West.
It’s typography, not orthography. It’s not a diacritical mark. It’s just an alternative means of showing an abbreviation. In that sense you might call it quasi-punctuation.
So, alternatively, a M[sup]c[/sup]Ron. 
That is also my experience. Well, except for what my first grade teacher tried to “teach” me during the first hour of the first day of school; that bint told me I was misspelling my own family name and that it should start with “ma”. I got in trouble when I told her where to go and what I thought of her family name.
Where’s that little devil’s horn emoticon when you need it?
“Bint”? 
You don’t want to know. 
Arabic for girl or daughter, taken, apparently into English as a derogatory term for (unpleasant?) woman.
England only? Is it commonly understood there? Is it new?
Never, ever heard of it.