That's the clean-out for the spark arrestor.
It's removed, normal end plugged with something, like a gloved hand, and the engine reved a few times to blow any carbon buildup out of that now open port.
If it hasn't ever been removed, chances are the bolts will break off in the muffler body, then need to be drilled out and the hole threads repaired and new bolts and gasket used.
A tip on those bolts is to cut them to a length where they only tighten in the threads and not beyond them. When the bolt protrudes past those threads, the exposed end becomes covered in exhaust emissions and makes removing them that much more difficult.
The story of three wheels and a man...