BLUF: small rectangular cube (cuboid) a little less than half an inch long.

I was attempting to modify my stock with a folder when I found an extra part on the work bench. I didn't know where it came from (assuming lower receiver since that what I was working with). It's not a part of the folder mechanism which I ended up not using- too much play. I've looked over diagrams and can't find anything that resembles a long metal cube. (And I have no other modifications to the lower.) Thanks for any help. And if I can figure out how to attach a picture I'll upload one later.

It goes on the receiver extension end plate.
like this one

Thats my guess.  Them little suckers are hard to get in always gave me trouble.

I only found one. Are they required for operation? I’m going back to a normal end plate also - are they only for the magpul sling attachment?  Thanks.

Thanks!

The kit comes with 2, one for a milspec RE and one for a commercial RE, so only one is used.  On a pistol RE tube, without the receiver end plate notch, they are not used at all.  If you do not use them on a RE that does have the notch, or can even happen on a pistol RE without the notch, if you are not careful tightening up the castle nut, the REP can ride up and interfere with the CH.  I had that happen once on a pistol and it caused a moment of head scratching as to why my CH would not pull back.