Since my area of collecting is Constantinian period there are many previously unresearched coins to be had if you know what you are doing, which is why those that are in the know do not ignore it. For example one only needs to do a simple check on the 'not in ric' lists of specimens known to find that a great many come from ebay; http://www.notinric.lechstepniewski.info/av7.html Weirdly I use Ebay to purchase these, mostly.
Congratulations, Ro!!! Your Sestertius (Reverse: Diadumenian facing right with three standards = RIC 211, BMCRE 149, Cohen 7, Sear 7453) belongs to Macrinus´ and Diadumenians second emission and was struck at Rome between late July 217 and late February 218. It is very rare: There was only one single Sestertius of Diadumenian (of your type, by the way) found in the Guelma hoard with two of Macrinus and 7.484 Sestertii of other Emperors
Julius-G => congrats on that cool Diadumenian OP-addition (it's a sweetie) ... this is my only example of that fella ... Diadumenian AE19 ... "eagle" in exergue (yah, that's why I bought it)
Nice coin! A sestertius of Diadumenianus has been on my list for a very long time to go along this as of his as caesar under his dad Macrinus: RIC 212 (Macrinus)