So why? AE20mm 3.20g, billon antoninian, Viminacium mint, sixth emission, 256-257. IMP GALLIENVS PF AVG; radiate, draped cuirassed bust right SPES PVBLICA; Spes advancing left, holding flower and raising robe cf. RIC 403F (Mediolanum), cf. Gobl 837, Cunetio 0784, Stevenage 547(J), S The short answer would be: it's such a quintessential product in silver billon from the (previously) provincial mint of Viminacium in Moesia Superior. Viminacium is known in ancient numismatics as the place where the large series of COL VIM bronzes was from, starting with 239, when Viminacium was raised to the rank of Colonia to 253, around or soon after Valerian raised his son Gallienus to the rank of Augustus. But the story of Viminacium as a mint does not end with the provincial coinage for Moesia Superior, but rather develops into a rather important Imperial mint, striking not just silver but bronze too, without the SC senatorial marking. The bronze is very rare and was probably issued for a brief period soon after scraping the provincial operation, but the silver antoniniani had 7 emissions, from 253 to ca. 257, although the mint could have stopped issuing by 256, as it did not mint for Valerian II -- see more about the emissions here. The series of Viminacium antoniniani is marked by a rather schematic style -- see as best example the representation of Spes on this specimen's reverse -- a feature that would follow the issues of this series. Also, I think that this bust type, with drapery over the cuirass (I think you can see the shoulder at 7 o'clock just behind the beginning of the obverse legend) is not recorded for the type. Unfortunately the corrosion at just the "right" place makes it hard to be certain, although I am pretty sure that it is not a truncated bust with cuirass only as it was regular for the type on the account of the very visible and rounded draped shoulder just mentioned. Similar specimen, but with the usual bust type, cuirassed only, here.