I know exactly what it is, I have one with my Singer Featherweight machine that I got in 1942. I is a ruffle attachment. For the life of me tho I can not remember if you had to attatch a smooth plate to machine before you put the attachment on. I made many many things with ruffels when I was in high school and I don`t know what I would have done without it. Guess I will have it get it out and check on it, I haven`t used that machine in years, I have one of the newer models now, and at my age no longer wear many ruffels.