Hi, can't really see the grey attachment properly but I think it looks like a tucking (doofy) to make little tucks instead of gathering. If it is, it makes a really pretty effect sort of like on a baby's dress when the yoke is attached it has little tucks instead of gathers. It that is right it fits on the screw on the needle bar and moves backward and forward in front of the needle and goes up and down to make the tucks. Was a long long looooonnnng time ago. Hope this is helpful.
If this is not what I am thinking of then I don't have a clue.
Sorry about that. Gleniveve.