I've been reading William Manchester's book "American Caesar", which is about Douglas MacArthur, and I found that he uses a strange convention for pluralizing the family name. When talking about the MacArthurs as a whole, he writes MacArthur' with an apostrophe, as in "After the war, the MacArthur' lived in Tokyo while the general was proconsul" (yes, he uses that term to describe him).
I have never seen or heard of a rule that would prescribe this. Manchester is a bit old-timey in his style: for example, he also writes "in behalf of" instead of "on behalf of", which is the only one I have ever seen. So perhaps this is similar. Where does he get this apostrophe from?
Edit: There seems to be some difficulty finding examples, which is odd. Here is a direct link to a page from Google Books that shows the apostrophe.