I am extremely fascinated by this piece and I have a chance to buy it.
It was sold by an italian auction house a couple of years ago and described as:
Contorniate, Minerva type, Rome, c. half IV - V century AD, AE, (g 25,95, mm 36, h 12). Helmeted bust of Minerva l., holding sceptre, Rv. Roma seated l. on cuirass, holding Victory on globe and reversed spear; in field, S - C. Alföldi, Kontorniat, pl. 24,1-7 (obv) and 214, 2 (rev); C 9 (obv).
Very rare. Apparently unpublished. Hole at h 10/1. Dark patina. About very fine.
I am not convinced that it is a contorniate, but rather a medallion struck for the inauguration of Constantinopolis.