Unfortunately, the metal visible under the corrosion products is no clue to age. The forgers are experts in creating artificial corrosion, from a slight surface pitting to near complete replacement of the metal with corrosion products. I have several such forgeries in my collection.
Ancient casts are interesting. During the tetrarchy there were real factories producing cast coins in clay molds. I have seen hundreds of such clay molds, but very few of the casts thus produced. Presumably the casts went to the melting pots when found by the authorities.
During the period of concern here, around 318-321, very few such casts were produced, if any. There are no such clay molds to my knowledge. Instead, the forgers produced dies and struck coins. They either used their own metal, or when possible, the “illegal” coins of Licinius. When Constantine and Licinius fell out, coins produced in mints under Licinius ceased to be legal tender in the west. They basically became worthless. But the forgers used them as flans, striking western types with their new dies. Such overstruck coins are common.
This particular coin could theoretically be an exception, a cast produced at about the time of the original. But in my view, the likelihood is far greater that it is a modern product.
