Antoninus Pius. 40-as

Antoninus Pius; 138-161 AD, Rome, 139 AD?, As, 10.81g. Cf. Strack-886 (COS II not III on rev.). Obv: ANTONINVS - AVG PIVS P P Head laureate r. Rx: TR PO[T] - COS II on l. and r., S - C across field, Janus standing, holding vertical scepter in r. hand and grasping drapery at hip with his left hand. Ex Curtis L. Clay Collection; ex Lanz, eBay, 9 Sept. 2011. The asses and sestertii of this Janus type are otherwise always dated COS III, 140-144 AD. Does our coin with just COS II (139 AD) show that this type was already being officially struck in 139, or is it just a mistake of an engraver working in 140-144 who wrote COS II, leaving out a stroke, when he meant COS III?. aVF