Constantine I, AE follis, Lyons mint. AD 306-337. 26 mm, 6.25 g. 

FL VAL CONSTANTINVS NOB C, laureate, cuirassed bust right.
MARTI PATRI CONSERVATORI, Mars, naked but for helmet, standing right, holding spear and resting left hand on shield. 

N in left field. 
Mintmark PLG.

RIC VI, Lyons 240 var (obv. legend); Vienna 78932.

Image courtesy of the Muenzkabinett, Vienna.