[tubman]

That's in the heat riser passage from the exhaust system. It looks like a hole got put into it, and someone tapped the hole and put a bolt into it to plug the hole. There isn't supposed to be a hole of any kind there. The only thing I can think of is that someone at some time decided they wanted to run an automatic choke and put the hole in there for a tube to supply heat to an automatic choke. When they found out it didn't work, the decided to plug it. If you have access to someone who is good at welding aluminum, I'd have it welded up. If you're cheap like me, I'd just clean it up and patch it with a high quality aluminum epoxy. There's really no pressure there so it would probably work fine. (Make sure what you use is intended for high temperature use.)

It looks like whoever had it modified it for a PCV system instead of a road draft tube as well.