Well here is what I think it might be used for , just my thought.I'm talking many years ago , when I worked in a single needle machine in a factory, there was a foot we used, to feed thin elastic throught the foot part between the two metal parts and run the material under the foot it self, which made a zig-zag movement and attached the elastic to the sleeve or whatever the part of the garment needed elastic.It's been a lot of years since I worked in a garment factory, but I'm thinkin that is what the foot can be used for.The screw part was for ajusting the tension of the elastic running through the two metal parts and would give the drag it would needed to pull the material in under the foot. It's hard to explain without showing who the elastic gets feed through.The elastic would go between the two metal pieces,out the front and then ,in under the foot,through the retangle opening, and as you sew the elastic gets feed through the opening and attached to the garment and as for the other arm attached on the right side is like a pressure arm you can press to ajust pressure on the garment.That is my opinion.