The ones from ancient Egypt were for polarizing the body. One was copper and the other zinc so that when they were held, they literally turned the body into a single cell storage battery. It also contained various crystals and minerals ( used as orgonite? ).
While the historians did not have a clue, these are frequently seen in many hieroglyphs being described by historians as being scrolls in the hands of the Egyptians. If you look closely, they are not scrolls.
The metal cans we believe to extend life and health. There are some extensive documents written about this although the idea is rejected by many authoritarian sources.