My 1956 Mercury sounds like an old tractor, so I've been chasing exhaust leaks.  Driver side exhaust manifold was completely missing the gasket, plus half the manifold bolts were only finger-right.  I thought for sure that would solve my problem, but there's definitely another exhaust leak, right around this doohickey in the attached picture.  Looks like some type of heat riser?  Obviously not working, the lever just flops back and forth and it's not hooked up to anything.  Looks like a nail shoved in there at the top, and also looks like it might be cracked or broken around the manifold.

Can someone explain what this is and how it's supposed to work?