Virgil,
Good questions.
We can't be 100% sure as we have no comprehensive
mint records, but there is no evidence at all to indicate that any official late
Roman mints made bronze coins by casting. Simply none. By contrast we have many mint-related writings and images that do show striking or talk about people involved in striking.
And as
Peter noted, if you have the
mint making them by striking is easy and efficient.
But when they appear is such volumes they are certainly indicating something. And
Egypt was totally stable at this time - the situation was not like politically divided
Gaul in the late 3rd century, so the
cast coins are not from some sort of separatist entity.
That is why I spoke of the spectrum of fully official to outright
counterfeit and the situation in
Egypt was probably closest to, but not actually at, the official production end.
Personally, I think that they are too common in 4th century
Egypt to just be a local private phenomenon. I think they may have been sponsored, or at least requested, though maybe not made, by regional officials. But that is just a guess.
SC