Here is an early issue of the MARTI type, struck shortly after Constantine defeated Maxentius. This type from Rome is more scarce than from the other mints.
Constantine I
AD 312- 313
22mm 3.3gm
IMP C CONSTANTINVS P F AVG, laureate and cuirassed bust right.
MARTI CONSERVATORI, Mars standing right, head right; right leaning on reversed spear, left on shield.
In ex. RP
RIC VI Rome 307