A thought to confuse the issue even further... The crews of ships were often called by the name of the ship, or a derivative thereof, as a kind of rallying call to differentiate between ships when seamen were on shore... especially in warships. This habit would be carried over naturally from naval to merchant ships. So, could the "Ruler kings" be the crew of a merchant ship named, for example, Sovereign, or Queen or King somebody or other, as a reference to themselves as the crew of that particular vessel, and as bloody fine seamen as well? As good a theory as any other.....