Here is one other piece of info - from the
CNG catalogue sale in Jan 2006, on why this is so
rare:
"The coin's
attribution to
Siscia along the northern frontier is also important. Upon
his proclamation as emperor by
his troops following the murder of
Aurelian,
Probus advanced toward
Rome. While making
his way there, he and
his forces defeated the Goths and Germans, and refortified the empire's northern boundaries. During these early days,
Siscia would have offered a strong central base from which such operations could occur, and from here the start of a new imperial court could begin. With Probus' assumption to the consulship bolstered by these early successes, it is quite likely that the ceremonies surrounding this event included an epic panegyric (over which the Muse Calliope would have presided), and the distribution of special coins to those in attendance, thus accounting for this issue's extreme
rarity and lack of
mintmark."