What does the switchblade Heck Tate uses have to do with anything? Did he switch the knife?

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
kitchen knife), and then a scratchy face.

So, Heck finds
Bob stabbed with a kitchen knife, placed there by
Boo.

Later, Heck Tate shows Atticus a switchblade he claims
to have taken from a drunk man that night.  That drunk man was Bob Ewell.  He obviously
didn't use it against Boo or the kids as there is no blood on
it.

Heck's a little slow, sure.  Why he didn't let Bob
sleep one off in the town jail is beyond me, but luckily he took the knife before Bob
could use it against Jem and Scout.  Bob no doubt had planned to use it in his plan to
get revenge on Atticus for defending Tom and embarrassing him in
court.