What if anything, do all the items left in the knot-hole have in common in To Kill a Mockingbird?
The items in the knot-hole are all symbols of childhood. By sharing them, Boo is getting in touch with his childhood self. He is childlike, and is meeting the Finch children on a child’s level.
All of the objects in the tree have value only because they are things children treasure. A spelling medal, an old watch, and chewing gum are basically meaningless. Yet they are treasures to children.
When Scout stands on the Radley porch at the end of the book, she imagines what it must have been like for him. She sees the events of the book from his perspective.
They stopped at an oak tree, delighted, puzzled, apprehensive. (ch 31)
At the end of the story, Scout notes that Boo gave them many things, but they never gave him anything.
Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies, and our lives. (ch 31)
This is not quite true. In giving them his childhood treasures, Boo got to imagine he was a kid again. He got to delight in their joy. He got to feel a connection with other human beings.