It is an adjustable zipper foot. I have seen adjustable zipper feet that one can sew with the needle on either side of the foot or the middle.
http://sewing-machines.blogspot.com/...pper-foot.html and
https://www.ecrater.com/p/23544215/l...able-wide-body I think J Miller had one, but couldn't find it.
I have not seen one that could be used with the quilting bar guide like yours, until I found one this morning that I think some of the pictures have it put together wrong and has the guide attached in a different position .
https://www.ecrater.com/p/23544215/l...able-wide-body The quilting bar guide would go through the U-shaped part and then tighten the screw (the screw facing you when the foot is attached) at the position that you want.
Janey - Neat people never make the exciting discoveries I do.
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