I think the slotted foot goes with the Singer faggoter (sp?). There are 3 pieces with this item--the bar where you can wind thread around some prongs, the foot that goes over the prongs (which I think you have), and a sliding thing on the bar to help get the thread off the prongs. It is used to make decorative lace-type things for clothing, etc. The foot is one thing that seems to be missing most from the pieces I see on Ebay. Go to
www.april1930s.com and the manual section to see what the faggoter looks like. They are very expensive on Ebay--I've seen one they were asking $325 for but it was the complete set.