It has to be
Parthia, of course: she's the only province to hold a bow and quiver resting on the ground behind her in that way, and the R of her name is visible at 11 o'clock.
Not the rarest province in the series, but not the commonest either; all of course are
scarce.
Strack 791-2 cites specimens in
Paris (2), BM (2),
Vienna, Naples, Bologna, and
Feuardent (dealer in
Paris).
It's interesting that
Parthia, a foreign country, is shown donating crown gold to Pius on
his accession.
Strack p. 42: "We know from other sources that client kingdoms did indeed pay crown gold," with citations in note 102