Rare Vintage Singer Model 221 Featherweight Sewing Machine Left Toe, #15429... Back in the day... Thefunction of this attachments was to allow the needle to pass close to the edge of a cord placed under a folded-over piece of fabric. This decorative finish can be found on many pillows and cushions, and is useful for other sewing projects as well. I'll bet zippers hadn't been invented when they first designed this foot, though nowadays this foot also allow a seamstress to sew close to the edge of a zipper during garment construction.
This is a left toe, #15429. This foot came from a closed Singer Company Store closed in Fairmont WV. These will also fit all of Singer's Vertical Shank machines such as 15-90 15-91 201 .