Most ruffler/pleater feet have a thin piece of metal that moves back and forth to push a small amount of fabric under the needle and this foot does not have that. Also a ruffler/pleater foot does not have a side-to-side or zigzag motion like you said this foot has. I think it is either a simple zigzag or blind hemmer attachment that would go on an old straight stitch machine. Singer had a zigzag attachment that had different patterns, similar to what the cams that came with my old Kenmore machine had for simple zigzag embroidery patterns. It would make sense that the machine companies would have come out with an attachment that could do zigzag and/or blind hemming with their straight stitch machines. Boy, have our machines come a long way since then.