The little disks are the keeper buttons, one per mouse: a sandwich of magnet and washer, with a leather ring for padding and a picture of the Salamander to watch over them.
Four-Spot scuttled off to a new home on his first evening in the world and Ghostie and Cousin Ratitia left the day after. Scamper and Cooper have homes now too, and so does Candy Man (at last: May 2018). However, if none of their younger cousins in the menagerie squeak to you, you can write to the Salamander to discuss construction of a mouse of your very own.