These are the same sewing machine head hinges used on our treadle reproduction sewing cabinets. The large 1” round head is mortised into the back side of the cabinet sewing bed top. This allows the weight of the machine to be evenly supported around the head. The screw merely holds the hinge down into the mortise.
These hinges ship without the screws due to different thicknesses of cabinet tops they are being used on.