Constantine I AE Follis. London mint, AD 312-313. 

CONSTANTINVS P AVG, helmeted, cuirassed bust left, 
holding spear and shield.
MARTI CON-SERVATORI, Mars standing right, helmeted and 
in military dress, cloak spread over upper chest, holding 
upright spear, point downwards, left hand on shield. 
Star in left field. 
Mintmark PLN. 

RIC VI London 258 var (bust type); BMCRE 1012.1.
Very rare.

With permission of Roma Numismatics, March 2014.