The following coin of Severina is listed properly as a billon antoninianus. That means the proportion of silver composition is nearly 30 percent. However the exergue reads XXIR, suggesting that it was struck in Rome and that silver proportion is 1 to 20 (nearly 5 %). Is there an explanation to this? . There's a small letter gamma to right field of reverse to symbolize the third officina. Concordia Militum as legend on reverse. Weight 3.48 g.
The term billon is used rather loosely in this hobby, referring to any silver-containing but predominantly copper alloy. Yours is a rather nice example of that coin, RIC 4, RCV 11705, from the Rome mint. Here's mine:
SEVERINA Antoninianus OBVERSE: SEVERINA AVG, diademed, draped bust right on crescent REVERSE: PROVIDEN DEOR, Fides with standard and Sol with globe standing facing each other Struck at Ticinum, 275 AD 3.6g, 22mm RIC 9
