What does Marcellus give Tova, and where has he seen another key like it?

Marcellus gives Tova the house key that she lost on the floor near the place where she stores her things when she cleans. He found it on the floor and took it and stashed it in his den with his collection of other things. He had seen a copy of the key when he was very young and before he was captured. It was attached to a circular ring at the bottom of the sea near a dead human.