This is the Vintage Original Screw for the Bobbin Winder Tension Bracket.
The Bobbin Winder is designed to wind the thread onto the bobbin smooth and even - with the thread flowing side-to-side as it winds. If your bobbin is not winding evenly, it very well could be the position of the Tension Assembly Unit. Unscrew the screw that holds the bracket in place and adjust it slightly to the left or right for even flow of thread to the bobbin.
Bobbin Winder Tension Bracket not included but can be purchased separately.