Origins of the Mizpah (sp)

I was just wondering what the origins of the Mizpah were. I was once given one where I had half and my then girlfriend had the other half. I understand the significance of the 2 halves separated, but was really curious as to the origin of this little device.

Here’s a picture of a Mizpah coin, for those unfamiliar with them.

I’ve heard that the given of Mizpah coins was a tradition that started sometime immediately before or during World War II. Sorry, but I don’t have any more information than that.