In Chapter 30, Sheriff Tate is having a discussion with Atticus about Bob Ewell's death. Heck Tate is handling a switchblade during the conversation that Lee suggests belonged to Bob Ewell. However, Tate explains to Atticus that Bob Ewell fell on his own knife and the switchblade that he is handling did not belong to Bob. When Atticus asks Tate whose switchblade he is holding, Tate tells Atticus that he picked it up off of some random drunk citizen downtown. Interestingly, Bob Ewell is found dead lying on top of a kitchen knife. As was mentioned in the previous post, Bob Ewell more than likely was armed with a switchblade while Boo Radley was armed with a kitchen knife. Tate wanted to cover up the fact that Boo Radley saved the children so he removed Bob's switchblade from the scene and said Ewell fell on his own knife.

The piece of evidence that Heck Tate tampers with, after Bob Ewell attacks Scout and Jem the night of the Halloween pageant, is a switchblade knife. We are not told directly that the knife is directly connected to the crime, but the implication is clear. When Heck proposes to demonstrate how Ewell fell on his own knife, he removes a long switchblade pocket from his pocket. He flicks it open and mimes the crime for Atticus, then clicks it shut and puts it back into his pocket. 

The conversation has already moved on when Atticus says, "That was a switchblade you were waving. Where'd you get it?" 

"Took it off a drunk man," Mr. Tate answered coolly. 

Scout is lost in her thoughts, trying to remember what happened, being confused by the dialogue, when Atticus says, "Heck?" Atticus clearly doesn't believe Tate's story about the knife. Heck responds: 

"I said I took it off a drunk man downtown tonight. Ewell probably found that kitchen knife in the dump somewhere. Honed it down and bided his time... just bided his time." 

Atticus drops it, but it's clear that Ewell came armed with the switchblade and Radley had the kitchen knife.