eBay: Maximianus Potin Tetradrachm - Year 8 billon tetradrachm of Maximianus, minted in year 8 (292/3 AD) at Alexandria. The reverse is an eagle, standing left, looking back, with a wreath in its beak. My online research says that it's probably made of potin (an alloy with no silver), not billon. It's about 18mm in diameter, and 4mm thick.