I just saw this coin in a lot and thought that I would post it. Besides being rare (r5 in RIC) it has the cryptogram in the mintmark. If you are unfamiliar with the Eros mintmark, here is my page- http://www.constantinethegreatcoins.com/ROMAE/
Constantine II
A.D. 320- 321
18x19mm 2.8gm
CONSTANTINVS IVN NOB C; laureate, draped and cuirassed bust right.
CAESARVM NOSTRORVM surrounding laurel wreath enclosing VOT V.
In ex. R EROS (in Greek) Q
RIC VII Rome 231
The seller would not ship to the US, so somebody got a good deal...the lot sold for 25 euros. Below are the original pictures.