What does this washer belong to???  

What does this washer belong to???

Hi,

I mounted my new Prusa XL (2 extruder version) one week ago and only yesterday I started my first print. I had some issues of collision during toolhead change so this morning I've loosen a bit the belts following the prusa.io belt tuner procedure. 

Today I got another docking issue while switching to extruder #2. This time, the blocking was being produced by a washer "floating" between the dock top magnet and the extruder #1. I don't recall the origin of this piece, nor having removed any other that could lead to a washer coming off.

I attach a photo of the washer. Any idea where this could come from? I'm really concerned about loosing random parts of this brand new printer. I performed the docking calibration, the silicone seal height calibration and the tollhead calibration (with the piece inserted in the middle of the bed), but to my knowledge none involved the mysterious washer.

Thanks in advance!

Sergio