This mint mark is a cryptogram, and is Greek for eros, which in Latin is amor. This in turn is a palindrome, as amor spelled backwards is Roma.
page about this cryptogram (http://www.constantinethegreatcoins.com/ROMAE/)
Constantine I
A.D. 320- 321
19mm 3.2gm
CONSTA-NTINVS AVG; Laureate head right.
DN CONSTANTINI MAX AVG laurel wreath enclosing VOT XX.
In ex. R eros (in Greek) P
RIC VII Rome 225