This rattled and then shook out of the inlet side of my stock pipe.  Can someone tell me what this is?

photos.app.goo.gl/hDNGXQ9Bue23RTi16

Looks like it could be part of an old exhaust seal.  Does it fit the exhaust port?

It's a metal band that was placed just inside (1") the pipe opening, and just underneath the first welded joint on the pipe.
Kind of a alignment sleeve / support ring that kept the two pieces in place as it was being welded.

Normally this ring is solidly attached by the weld melting through the pipe joint, and into the ring itself.
The ring is not necessarily needed if the weld is a good one and shows no sign of leaking.

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the piece's outer diameter is a little over 28mm. The exhaust gasket inner diameter is about 36mm, which suggests that it is not part of an old seal.

Thanks Dave. That is a relief.