Roman number does not indicate the order of discovery. It shows the order of isolation of the limited enzyme from the specific strain of a microorganism. Since Hind-II was observed earlier than Hind-I, that is why the first discovered restriction enzyme is Hind-II. But Hind-I becomes first isolated from that strain of microorganism from which Hind-II becomes isolated later. That's why there is II within the first found endonuclease enzyme.