what is a Kensington Attachment? When I did a google search, it looks like some sort of clamp for electical wires.
Since you posted you have a coffin top machine, it can't be the cable for the machine lift as that does not have one. I've got about every attachment sold to go with the Domestic machine (and duplicates of most) but I don't have one called Kensington, so I'm real curious what it looks like.
BTW- eBay is the only place I look for vintage sewing stuff. Etsy also has some, but I find their search methods difficult, and their prices higher than eBay.