A very nice piece.
Obverse: bust of the emperor in laurel wreath, armor and paludamentum, to the right, in the rim legend:
IMP CM AVR SEV ALEXAND AVG
Reverse: Pax standing left, holding olive branch and scepter, surrounded by legend:
PAX AVG;
Silver, diameter 20 mm, weight 3.05 g, axis 11 h.