Daylesewblessed is correct. You need a buttonholer in a green case to fit your featherweight. The older black buttonholers come in a rectangular green case with a snap closing. The more modern ones are tan and were sold in an oval lighter green coloured case. The red or rosy pink cases were buttonholers for slant shank machines (ie 301). The basic buttonholer came with 5 standard sized cams but more could be purchased. Earlier cams were all metal while later models included plastic cams. The buttonhole cams are interchangeable and will fit in any of the buttonholers. The buttonholer also includes a plate to cover the feed dogs and a big screw to hold it on.