If it's like this one and hasn't been switched out, it's for slant shank machines: 301, 401A, etc.
http://www.ebay.com/itm/Singer-Sewin...QAAOSwBahUziyO
I've heard them called the Jetsons case. It comes in a green version for low shank machines too.
No affiliation with seller, just the first picture I found.
Rodney