I have around 150 or so
Probus from
Siscia but am far from an expert but I can say that the
bust style doesn't match the early emissions (first and second), which were more like the following example:-
and these were the only ones that used the Greek
officina scheme A, B,
etc., for
Siscia in this way. As G/< mentions, the
style of your coin is more typical of the much later emissions.
The P/B is an oddity, the lower
part of the B seems badly formed and was surely intended as a P, whether some fors of die break could have malformed this into a B or whether the engraver made this error, we will probably never know. A nice and interesting coin nonetheless.
Regards,
Martin