I've found another style - the plate is closer to square shape.
At least I'm pretty sure this is the chain stitch attachment - the item in the yellow box
This is the long, narrow one
At least getting the attachment should be pretty easy - I've found most of the Kenmore accessory sets contain it. I find it interesting that there are so many attachment sets out there for Kenmore 158 series machines, but finding the machines is a lot harder.