Tacitus AE Antoninianus, Siscia.

Obv: IMP C M CL TACITVS AVG, radiate, draped cuirassed bust right.
Rev: PAX AVGVSTI, Pax standing left, holding olive branch and 
transverse sceptre.
Q in right field.
Mintmark XXI

RIC V-I, 187.

Courtesy of Galleria Antiquarica, 2007.