This is a genuine vintage Singer Attachment - the Cloth Guide or sometimes referred to as either the Seam or Hem Guide. This one is still in its original packaging and is rare to find!
The cloth guide attachment is a help in straight stitching. The guide is fastened to the machine by means of the thumb screw (included) into the bed of the machine as shown in the photograph. Attachment can be adjusted to various distances from the needle as desired - perfect for clearly establishing the 1/4" seam allowance for quilt piecing!
This attachment fits the Singer Featherweight 221, 222K and all other vintage Singer Sewing Machines (i.e. 15, 27, 28, 66, 99, 185, 127, 128, 192 Spartan, 201, 206, 221, 222, 237, 306, 319, 320, 328, 401, 403, 404, 500, 503, as well as many other brands besides Singer).