I found this prisoner I.D. tag with the remains of the fallen vanguard. I can only assume that all of their prisoners are tagged in the same manner.
I'm going to start cataloguing the tags in hopes of making sense of their penal system. If in your adventures you find more Ethereum prison keys, you can use them to open the Ethereum prisons at the Ethereum Staging Grounds - directly south of here. Bring me back any and all I.D. tags that you find.