Where does this part go???

It's a bit embarrassing but as I was taking apart the housing on my FS 45 string trimmer a part fell out. I heard it come out but didn't see where it came from. :confused2:

Stihl FS 45 part.JPG

Anyone know what it is and where it goes?

Thanks!

Attached Images

• Stihl FS 45 part.JPG (98.4 KB)