Hi all! I recently posted about buying a nice coin of Magnentius from eBay, with the pictures being very poor quality. However, upon closer inspection of the coin, it appears to be a very rare follis of Magnentius' brother, Decentius. I've tracked down one other example, which was sold for $650USD by Gemimi Auctions in 2014 here. I think this coin was expensive because it proved a hypothesis that Decentius had coins minted in Ambianum. Quoting Numisbids: "Our coin is apparently the first known of this variety for Decentius at Ambianum, confirming the hypothesis of Bastien 128 and Late Roman Bronze Coinage 16 that such a coin should exist. Mint State Since this type and mintmark are definitely known for Magnentius, they could be expected to occur for Decentius also. RIC 26 cites such a coin of Decentius from the British Museum, but probably in error, since Bastien 128 projects the type only, and could find no actual specimen anywhere" It also appears the two coins share an obverse die, although the reverses differ slightly (looking particularly on the ties of the wreath). My coin also has a slight die break on the obverse. My coin: (Since taking this picture, I have carefully removed most of the green seposits, especially on the obverse) The Gemini coin: I have no idea what my coin might be worth now, as I'm not sure how the market has changed in the last 7 years. If anyone has any information on this study, and why it's unusual for a Decentius to be minted by Ambianum (or the coin's possible value, given that it probably won't be staying in my antoninianus collection lol), I'd be very grateful to hear it! Thanks!
Note that the quote you supplied is not that Decentius did not mint coins at Ambianum; but only about this specific mintmark AMB dot in crescent.