I don't have an answer for your question, but I've never seen one in person or on the web. If there were some made, they must be few and far between.
I've bought two after-market (new) walking feet for my slant machines (503, 401 and 778) and both have worked just fine. One was labeled as a Singer and the other has no label.
I've found you have to reduce the pressure on the presser foot, reduce the tension and elongate the stitch to about an 8.
bkay