What is this small piece that fell out of my S8?

I was right in the middle of a full display with frame swap and while I was unscrewing the charging board, I turned the phone over facing down to try to get a screw out and I noticed two pieces fell out. I knew it wasn't two screws because I only do one at a time. When I looked, it seems like some kind of pulley system or rolling part, See picture. The black part feels and looks like plastic while the gray part feels like rubber. I've never seen a part like this and I'm a little OCD about having spare parts at the end of repairing a phone that has never been opened. What is this thing? Please help!