Constantine I, AE Follis, c.313-314, 

obv: IMP CONSTANTINVS AVG, laureate, cuirassed bust right
rev. MARTI CON-SERVATORI, Mars standing right, holding 
reversed spear and leaning on shield.
S-F across fields.
Mintmark PLN

RIC VII London 4. Rared R4.

very rare, green patina, almost extremely fine 

Estimate £ 110-150 

Courtesy of Spink & Co. Auctions, April 2010.