Rare Singer Model 221 Featherweight Left Toe Foot #15429 that will set off your vintage Singer Sewing Machine.  Back in the day... 

The function of this attachment 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 these feet, though nowadays these feet also allow a seamstress to sew close to the edge of a zipper during garment construction. 

You are purchasing just the left toe that is pictured.  This came from a closed Singer Company Store closed in Fairmont WV.