My first Kenmore had this attachment. It was a pre-1980 machine. I think the screw on the left there attached to the machine. The disk on the right has the different sized/style buttonholes. The disk rotated so you could pick the size/one you wanted. I believe mine came with two different disks that you could interchange. Something followed the pattern of the buttonhole shapes on the disk. And the needle stitched through the oblong hole in the middle.