Use of lone apostrophe for plural?

Solution 1:

There are a couple of rules I've found which might explain this. One is that in Gaelic, dual masculine nouns are identical to singular, so you might have the MacArthur meaning one or more MacArthurs.

The second is the apologetic apostrophe. Old Scots writing never used an apostrophe, but people started putting one in to help correlate Scottish writing with its English equivalent (so wi' for with). It might be the case that this happened with the missing s in the MacArthur too.

Solution 2:

Is it possible that this was a search and replace error? Sometimes when a book is edited there might be an attempt at automated correction of a common mistake. For example, perhaps the author often forgot to include an apostrophe when meaning to write "MacArthur's." If an editor tried to search for this mistake and insert an apostrophe each time it occurred, it may have erroneously produced "MacArthur' " when the author genuinely meant to refer to the family as "MacArthurs." One such search and replace action alone might not produce the noted error. However, multiple passes of similar search and replace edits might lead to what the Ryan noticed.

Here is a clbuttic example you may enjoy: http://forums.thedailywtf.com/forums/t/5552.aspx