OK, so here I go, dipping into Blog-o-landia!

If I'm not mistaken, this is called a Ruffler Foot, probably for an OLD Singer. It's intended to make little "creases" in the fabric and advance it to the next crease. So the vertical "fork" holds up the intended ruffle, and the little "clutch" grabs a small bunch of the fabric and advances it over the flat stock.

Not sure exactly how to load fabric into this complicated looking foot(so now you know the answer to 'have I used it' -NO). But almost certain it's meant to attach a narrow, folded band - perhaps cut on the bias? - to a flat (and larger) piece, as in for example, around a collar or around the edge of a curtain.

That's my best guess (and I've also wondered, so I'll be happy to find out if someone really knows...)