Can anyone tell me what the part circled in red on the photo below is? It's on an 83 380sel that I just had a custom cat back system installed on.

After everything was all buttoned and sounding great, I discovered there is very small leak coming from somewhere near the top of it. The outside is surrounded by a sheet metal shield and the oxygen sensor screws into it. Thanks.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire