Lifer, that muffler came out for the Aeroworks mustang. It use to be listed just for that but now you can get it from Tower or Jtec. If you call Jtec and specifically ask for the Aeroworks P-51 mustang muffler, you'll get that muffler. I used it in my ESM 50cc 109.
Rohnt, the bolt issue with that muffler unfortunately is common. I learned of it from a friend and had to do the same. Everyone I've seen has had the same resolve.