Constantine II, AE follis, Rome. Struck AD 317.

CONSTANTINVS IVN NOB CAES, bare-headed, draped, cuirassed bust right, seen from the back.
PRINCIPIA IVVENTVTIS, Constantine, helmeted and in military 
dress, standing right, holding reversed spear and resting 
left hand on a shield at his side.

A in left field.
Mintmark RS

RIC VII Rome 91.
Extremely rare, rated R5.