I recently picked up an odd Trajan Decius antoninianus cheap on eBay that I was having a great deal of trouble attributing. It features PIETAS AVGG with Mercury on the reverse, not a type normally issued for Trajan Decius. Stumped, I contacted Richard at the wonderful Four Bad Years website (one of my favorites: http://sonic.net/~marius1/mysite/). Richard told me this was a mule, a Trajan Decius Milan mint obverse (IMP CAE TRA DEC AVG legend) muled with a Herennius Etruscus or Hostillian reverse (they both used the PIETAS AVGG with Mercury type). As you can see from the Four Bad Years website, there are a lot of oddities like this during this "four bad years" time period, especially at the branch mints. As far as I can tell this is unlisted. It might be a counterfeit, although I am pretty sure it is ancient. The metal and weight are pretty good for this era. Any others out there like this? Or other 3rd century mules or weirdos? Trajan Decius Antoninianus (250-251 A.D.) Milan mint IMP CAE TRA DECIVS AVG, radiate, draped & cuirassed bust right draped / PIETAS AVGG Mercury standing left, holding purse & caduceus. Unlisted mule; Her. Etruscus or Hostilian rev. with Decius obv. (3.63 grams / 21 mm)
The obverse style does not strike me as quite right for that series so I would consider the coin unofficial. Many unofficial ancients combine inappropriate dies. Last I heard, they found a die link suggesting the RIC 'Milan' mint was just a later Rome mint series. I can not recall who told me that and I can not say whether the existence of such a list is conclusive considering the possibility of the link involving an unofficial coin.
Yeah, it is kind of fishy - the obverse style is very crude, cruder than the examples on Four Bad Years site. The eye in particular has a kind of Egyptian-side-view look that I see on Antioch and other eastern mints. I'll probably never know for sure...