In this chapter of To Kill a Mockingbird, Scout is in costume: she's a ham, so she can't see or use her hands.  So, Scout feels around with her feet.  She pushes around on Bob's body with her feet: she feels a belt buckle, buttons, and "something I could not identify" (the switchblade), and then a scratchy face.

Heck finds Bob stabbed with a kitchen knife. This is the knife Boo Radley used to kill Bob Ewell.

Later, Heck Tate shows Atticus a switchblade he claims to have taken from a drunk man that night.  In reality, this is the switchblade Bob Ewell tried to attack the children with. Heck Tate pretends he got it from a drunk man earlier; he intends to make it look like Bob Ewell tried to attack the children with the kitchen knife and fell on it. The switchblade is evidence that the story he wants to tell—the story that saves Boo from daunting public scrutiny—isn't true, so he fabricates a new origin for it.