These knives are called butterfly swords (hudiedao). I'm no expert, especially when it comes to dating, but I think you are right, turn of the 20th century. They often come in pairs and are usually associated with Southern China regions, which may explain the River Priates moniker. In fact, they are a popular weapon within most, if not all "Southern" Chinese martial art styles (not that anyone practices with anything resembling the real weapons these days). There is a surprising amount of variation on the exact shape of these blades. Your example is probably the most common, while others may have long slender blades. Here are a number of examples from Oriental-Arms (I hope Artzi won't mind):
http://www.oriental-arms.com/item.php?id=2342
http://www.oriental-arms.com/item.php?id=1921