Constantine I. AE Follis. 19mm. Arles. 313-315 AD.

Obv: IMP CONSTANTINVS PF AVG, laureate, draped, cuirassed 
bust right
Rev: MARTI CON-SERVATORI, Mars standing right, helmeted, in 
military dress, cloak spread over upper chest, holding
 upright spear, point downwards, left hand on shield

Mintmark PARL. 

RIC VII Arles 23, extremely rare (rated R5)

From Dane's Collection, May 2010.