Yea I had some information about them somewhere I think. They were used in WWII as well or similar ones. The pilots could swallow them to keep them secure until they were through the searches and into the POW camps. They also had buttons sewn on their pants that could be used as compasses if floated in water. They usually had a mark to indicate North on the back of the button.
Oh I just found one of the ones I was thinking about its called the Peyser-SGI and are sold at www.bestglide.com which sales survival gear and other things to pilots ect.