That little circular piece sticks to a magnet, so it’s steel. I think it’s the end of the spring that broke off.
There’s no telling where that mangled ring came from — but whatever it is, it ain’t supposed to be there. Bren 2 owners are reporting all kinds of crud and swarf in their ejector channels, direct from the factory. Good quality control, there, CZ!!
From the two spare ejection springs I’ve been able to acquire from CZ over the years, yours appears to be more or less intact, though maybe somewhat knackered on my the one end where it’s double wound. Hopefully, you’ve discovered and removed the source of your ejection woes and hopefully you can get some fresh replacement parts…assuming you can navigate CZ’s crappy online Webstore.
It’s just a totally ridiculous situation with CZ and their lack of parts and service for these guns.