looks like an invisible zipper foot. They were normally sold by teh company making the zippers, and it came with several colors of plastic pieces to use on different machines.
You slide the foot over and the teeth of the zipper fits into one of the groves (depending on side) and it unrolls it just a bit so you can stitch right up next to the teeth.
Since I collect vintage machines and attachments, I've got a bunch of these now.
Back when I sewed bridesmaid dresses - I put in a lot of invisible zippers.