This is a vintage Singer buttonholer sewing machine accessory. It comes in the original teal blue case with accessories. I admit - I don't know much about sewing machine accessories. I can list out what accessories it comes with and answer any questions, though.
It includes the main piece, I assume the attachment, with a slide on the side with the following etched marks: W, 6, 4, 2, N. It is marked SINGER and has a knob on top. It also includes a T-shaped piece with a screw on the end (T bottom) that looks like it slides. It has five peanut shaped pieces that are smooth on one side and open on the other, with the opening surrounded by jagged teeth. There are two small screws similar to the one on the T-piece as well as a a long flat head screwdriver piece.
The case is a very cool shape and color. It stays latched close and has a few scuffs on it, but nothing major. I hope someone can use this piece or at least appreciate it's retro styling and use it as a cool decoration. Enjoy!