From what I have read recently:
- they were indeed either
sestertii or more likely double
sestertii- they were only struck/made circa 260-262
- they were
part of Postumus' attempt to create a coinage as
good as or better than that of the rest of the Empire
- they were struck at a time when the
sestertius and its
provincial equivalents were fast fading out of use thus
Postumus was grasping at a dying trend in coinage
- they may have been given a double-sestertii valuation by Postumous in recongition of the fact that their intrinsic value was shooting up when compared to that of the rapidly debasing antoninanii, however it soon became clear that the intrinsic value of the antoninaniii was falling so fast that even this double valuation of the setertius was hopeless
- the need for, and public interest in, this
denomination was, at least at first, so high that many imitations were made
- the huge
weight variance was due in
part to the fact that, like the antoninainus they were becoming a token, or largely fiduciary, coinage - thus
weight mattered less and less.
Shawn