Can anyone identify this part?  

Can anyone identify this part?

U.S. penny for scale. Sorry, I'm out of bananas.

I recently upgraded my MK3S with MMU2S to MK3S+ with MMU3. As recommended, I completed the MK3S+ upgrade first. This includes restoring the filament sensor in the extruder to the stock flattop (non-mmu) version. After a successful test print. I proceeded with the MMU2 to 3 upgrade, including reinstalling the "chimney"-style filament sensor. I struggled with the filament sensor calibration for a while, but everything seems to be working now.

This is where my initial question comes in. While cleaning up, I found the pictured part. Two of them, actually. I don't remember ever installing or removing this part, and I can't find it in the build instructions or printable parts lists. What is it? As I said, the printer and MMU seem to be working normally so it's obviously not critical, but it's driving me crazy.