From the desk of Stephen White....
initials used by the Acadians
The "S" initial your grandfather used did of course stand for "Ó Sylvain." This was a very common practice among the Acadians here in New Brunswick. There were often several individuals with the same first and last names. Among themselves they used "Damien Ó Sylvain" or "Damien Ó Sylvain Ó Sailor," but in written records, both English and French, that would be abbreviated to "Damien S.," or even "Damien S. S."
The initials do not always stand for formal names, but may also represent nicknames, as I suggested above in giving your grandfather a second "S." These nicknames sometimes referred to specific individuals, but sometimes they stood for an entire branch of the family. For example, in some census records I have seen the initial "C" used for certain LeBlancs whose fathers' names (and even their personal nicknames) had nothing to do with the letter "C." But I eventually realized that these LeBlancs were all descendants of Joseph LeBlanc dit Coudjeau, and that among the people at Memramcook they were known collectively as "les Coudjeau," and that is what the "C" meant. Needless to say, the initials can be very helpful, provided you can figure out just what they were intended to indicate.