Diadumenian, AV aureus, AD 217-218. 

M OPEL ANT DIADVMENIAN CAES, bare-headed, draped and cuirassed bust right, seen from behind.
PRINCIPI IVVENTVTIS, Diadumenian standing left, looking right, holding standard and sceptre; two standards to right. 

RIC 101.

Image courtesy of the British Museum.