A small aluminum knife is among the treasures that Scout and Jem find in a knot hole in a tree on the edge of the Radley property. The knife is on the chain attached to an old, non-functioning pocket watch.
Other items in the hole include two small carved figures that resemble the Finch children. It is possible that Arthur (“Boo”) Radley used the knife to carve these figures. More generally, the knife is directly associated with Arthur. The small knife is appropriate to a boy, approximately of Jem’s age, which shows that Arthur is identifying with the boy and perhaps giving him something that was meaningful to him in his own childhood.
The association of a knife with Arthur is part of the foreshadowing of his actions later in the novel, when he uses a larger knife to kill Tom Ewell. The small knife of a relatively weak metal, connected with his creativity, may suggest that Arthur is a peaceable person and that the later stabbing was an accident. But his having a knife also brings up his allegedly stabbing his father, as stated in the rumors that have long circulated around the town, which he did with a pair of scissors.