Joe is right. The attachments fit a 9W or W&W D9...they'll also fit a W&W 8. They're more of a "center bar" attachment than they are a "back clamp"....they insert into the presserbar and then are tightened down with a thumbscrew from the back.
Rare? No. They're a bit harder to come by than low shank Singer attachments, but they are out there and available.
