Recommended reading, at this link: Highly_Deceptive_Forgeries_of_Constantin
Abstract: “The most famous and sought after of all coins from antiquity include Constantine the Great’s SPES PVBLIC coinage. These small bronze coins were issued 321 -328 AD at Constantine’s new capital-to-be Constantinopolis. They are the first coins said to carry an unequivocally Christian message from the emperor to his subjects. They are hard to get, and a SPES PVBLIC is usually the star of any numismatic collection. Recently, a number of ingenious forgeries of SPES PVBLIC coins have come on the market. These forgeries have deceived every numismatist that has handled them, and they pose a threat to collectors and researchers alike. This paper examines these forgeries, to date the most skilled known of Late Roman Bronzes, and explains how they were produced.“