Test for the Mosin experts

Here is a test for the Mosin experts to see if you can identify this part. Rest assured, I'm not asking what the part is because I'm a newbie nitwit who just got his first pair of Mosin's, took them apart to clean them, put them back together, and found this piece on the table after I was done. Just testing you all.

It's likely it is part of an M27, but I'm not 100% sure.

Thanks for participating!

Ed