Hi Ignacio,
I agree it's clearly "CI", but the coin must be unofficial since the SARMATIA DEVICTA type was only issued for Constantine I.
The only DEVICTA type issued for the caesars (Constantine II & Crispus) was ALAMANNIA DEVICTA, which was only issed from Sirmium.
As well as the "CI", the bust style is a bit wrong, but generally a very nice unofficial coin.
Ben