Dan Martinic

It's the same flange in all three shots, just zoomed in further each shot. This is the downpipe end that the cat pipe bolts to; the cat pipe has what you describe (the groove to fit the seal ring). Cat pipe flange is near-mint, having bought a NOS cat not too long ago.

I did find some sandpaper--220 grit--and tried a little more. A little improvement, but not perfect. I went ahead and installed everything. Only one way to find out lol