That wasn't drilled. My muffler developed a hole one day in that same spot. I think it's a rusted out spot weld from when the muffler was manufactured.

I don't have a welder now that I'm retired, and the hole in my muffler was about the right size for a 1/2" fine thread tap. There's a lot of metal there so I ran a tap into the hole, cut a bolt so it had about a half inch of threads on it, put red loctite on the threads and screwed it in the hole. It'e been in there for three or four years now.

My muffler does have a couple of extra holes drilled in it, but they are in the end on either side of the actual outlet. Provided by one of my bike's previous owners . My bike has the stock jetting and it runs fine despite the holes. I don't know how much louder than stock it is with the extra holes, but it's still pretty quiet. The mechanical noise from the engine is at least as loud as the exhaust.

Thanks guys.  I do believe you are right, oldNslow.  

wow.