Very rare solidus with the mint mark SIS, in the RIC catalog the rarity is R4.
Roman Imperial
Constantius II (324-361), Solidus 355-361, Siscia mint
Obverse: helmeted and cuirassed bust of emperor facing, slightly right, holding transverse spear and shield decorated with floral pattern
FL IVL CONSTANTIVS PERP AVG
Reverse: Roma and Constantinopolis sitting opposite each other, holding a shield with the inscription VOT XXX MVLT XXXX, SIS in exerque
GLORIA REIPVBLICAE
Weight 4.18 g