I have the directions for the Singer Electric Sewing Machine copyright 1941 and in it it has a picture of an attachment much like this one. It is called a ruffler and is used to make ruffling, gathering and plaiting. I still have the attachment and you can also use it to sew piping on at the same time or sew the ruffle to the garment at the same time as well as add a facing.
Nana21