Is it Brainy's?? I'd forgotten about that. I just remember the ring.
Yeah, it is Brainy's. There's a two-part storyline in the original series where Brainy, Ultra Boy and Chameleon Boy go back in time to track Booster down. Once they see he's a force for good, they allow him to keep the items he stole (also because they think he has a role to play in how history unfolds and they don't want to disrupt that).