The stabbing sound effect from Halloween is really a knife stabbing what?

Being a Foley artist seems really fun. For one thing, you're basically just watching movies all day, and you also get to hit random objects with other random objects.

It should come as no surprise that for John Carpenter's original Halloween, they didn't actually stab people. Instead, they invoked the art of Foley, and stabbed this object. What was it?

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