The buttonholer in the green egg shaped box will also work on a Featherweight. It's just that the older style in the darker green rectangular box is black and will "match" your Featherweight better. The buttonholer in the egg shaped box was updated and is a light beige/tan colour. The buttonholers that won't fit a Featherweight are the ones in the maroon or dark rose cases as those models were made for slant needle machines (think Singer 301). Those vintage buttonholers in the dark green cases will fit any short shank machine and work very well on my 201 Singer and even my much newer Janome 6500 (don't like the programmed buttonholes on this machine) and I use my faithful old Singer buttonholer which is older than I am all the time.