According to
Hahn, in
MIBE, p.64, there is no clear answer.
He notes that there are also
rare versions of the
type from
Thessalonica with AISPI and AISP
.
He notes that several theories have been put forward to explain the AP:
argyron (currency) by
Grierson,
archaia rhope or antiquo pondere (referring to a
weight standard) in
MIB I, and
ad pecunium (plus S for
scrinium) by
Hendy.
However,
Hahn believes the the most probable is arca praefectura (plus I for Illyrici) denoting the office responsible for the production or provision of the metal, as suggested by
Kent.
The AP are also found on either
side of other
denominations:
, H, B and I.
The A is unlikely to be an
officina mark as no other letters is ever found. If
Thessalonica only
had one
officina at that time then why note the A, and if it
had more than where are the other examples. This is despite the fact that it is true that no
officina mark appears in the
exergue, which always has TES. But that was common at Thessaloniki, even in the 4th century. That is why
Hahn did not include it in the list of possibilities.
SC