Sorry, but I don't see a slip joint.  However, I do see what appears to be a plate welded over an apparent cutout in the exhaust flange.  What I would really like to see is what's inside.  

I follow you on the flat surfaces being a failure point.  That was one reason why I welded the inner tube to both ends... to keep them from flexing and eventually failing.  Time will tell how well it works. 

Mark S.

The black arrow points to a band of material wrapped around what I suspect is a cut through muffler.

The red arrows point to the edges of the band.

It is welded only on the right end.

The lite sheet between the flanges would be to prevent cooling air leaks into any number of ports into the center iron used to feed air into the stock exhaust ports to reduce HC and cool NOX.

This is all speculation on my part.

Lynn E. Hanover