The dish is the 3" Cat. The other is the exit pipe from the 3rd chamber through the 2nd to exit.
If you plan to weld it back up use the thinnest cutoff wheel you can find. This will minimize the gap you will need to weld. My suggestion would be to cut along the lines you see in my images for chamber 2.
On the other side (chamber 3) you will see a series of spot welds. Cut along the outside of these (you will see the discoloration ). This should allow you to remove the mesh screen with the fiber blanket attached. This will also give you direct access to the other side of the Cat. Something I could not do with my original cut.