oops. just remembered that my extra piece actually attatches to the top, and looks like it could be used for attatching trim or elastic.

yep. it's a zig zag foot for a straight stitch machine. the fork goes around the horizontal bar where your needle screw is attatched. I have mine right here in front of me (i've used it lots of times with my older Kenmore that I didn't have a zig zag cam for) and I have an extra piece that you dont' have pictured that attatches to the bottom. It has a piece that slides into the hole just above the needle hole. play with it on your machine it's fun to watch the foot work. :-)