I am fairly certain this is a ruffler. I have a Singer book from the early 1920's with a picture of a similar foot. The following parts are labeled: Foot (A)- attaches to the presser bar; Fork Arm (B) - placed astride the needle clamp; Adjusting Screw (C) - regulates fullness of the gather; Projection (D) - projects through the slots in the adjusting lever; Adjusting Lever (E) - sets the ruffler for gathering or making a pleat either every 6 stitches or every 12 stitches - also disengages the ruffler; Adjusting Finger (F) - regulates the width or size of the pleats; Separator Guide (G) - on the underside, contains slots into which the edge of the fabric is placed to keep the heading of the ruffle even. Also for separating the fabrice to be ruffled from the fabric to which the ruffle is being attached. Ruffling Blade (H) - upper blade with teeth at the end to push the fabric in pleats up to the needle; Separator Blade (J) - lower blade without teeth, which prevents the teeth of the upper blade from coming into contact with the feed of the machine or the fabric to which the ruffle or pleating is being applied.