Just changed my read diff. fluid for the first time. There was a semi-circular piece of metal stuck to the magnet. About the size of a dime cut in half. A little less than 1/8" thick. Looked kind of like a woodruf(?) key, but not strong enough or any wear lines showing. It didn't look like it got bashed around on it's way to the magnet. The differential seems to work fine. The collection of other metal shavings on the magnet were very minimal. Any thing to be worried about?

Is it tapered or curved or is it flat?

Seen that many times on those.

The piece is flat, looks like it was stamped, like the inside of something pushed out of a hole. I would say it is a fairly soft steel, not like for a hard wearing part.

I wonder if it wasn't left over from the filler plug hole?

I would say that's a pretty good call. Right size, right consistency of metal, bypassed the good stuff to get to the magnet. Thanks!