Nut shim, that's what I meant! Can a mod edit the title for me? God I'm such a fool.
Is it supposed to be like that though? That's what I was looking to find out. It seems a bit of an awkward way to do it, sticking out the sides. Plus it doesn't appear, as I said, to be a rectangle, more of an irregular shape.
I suppose taking it out won't do any harm (at least, none that I can't repair), so I might move it and see what happens.