This is a Bobbin Winder Protector. This will help protect the bobbin winder from getting a direct blow.
The idea here is there is curved area for what ever was going to hit it to slide off it.
I am sure if it was hit hard enough it could still break, but this just add another form of protection
This fits the New Bernina 4 Series, 5 Series & 7 Series (black bobbin machines , not 8 Series)