I agree that this is an unofficial coin.
However, on the question of half-denominations, personally I don't believe that that was the case.
Bastien (1983) continued to believe that there were half-denominations struck - in other words two sizes struck at the same time.
However, based on a study of the
hoards and the
mint marks,
Depeyrot (1992, Le système
monétaire de Diocletien à la fin de l'empire romain, in
Revue Belge de Numismatique et de Sigillographie CXXXVIII) disputed this and I agree with him.
I believe that what we are faced with is a that was slowly declining in size and a large output of undersized unofficial coins using similar
mint marks.
SC