While I have no idea what it is, I can assure you it's not something that would have come with the normal selection of Greist attachments, and it's not one of the more common add on peices either.
All kinds of neat stuff got stuck in with sewing supplies over the years - I've got some very interesting items that came inside drawers of cabinets.
If you are missing some of the peices that would have come with the set, send me a PM. I've got a bag of them that fit the White rotary machines, got some that fit the White VS machine too.