Wow!!! So many views, no answer.
Anyways, I fixed it: I pressed it back with a screwdriver wedged between the exhaust & the flange, and it snapped right back in.
Now I need to top up the differential fluid.

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Stable: e92is, e46 M54B25, e83 N52, e53 N62 - sold, e39 M54B30 R.I.P.