A nummus/centenionalis of this date (c.328) should be approx. 19mm, so the small size here is way off and indicates a half unit fraction - indeed quite a surprise given a lack of fractions at this date (previous
had been Req.
Opt.
Merit. of c.317, following would be
Pop Romanus - also approx. 12mm die - if correctly dated to the dedication of
Constantinople in 330).
Ben