Hi All

I've just had my sump off and when I gave it a clean I found this.
Sump find reduced.jpg
Anyone got any ideas what it may be?
I'm leaning towards part of an old oil filler cap.
It is about the right diameter as the existing cap with what appears to be a flat on one face.
The other face is where it was broken off with what could be the start of a thread.

Cheers Shane