It is called an "Attachment Foot" and comes in a few versions. It can be used by itself, for quilting or possibly with an underbraider which is attached to the bed. Frequently, I've found, that the Attachment Foot is missing in sets that have the hemmers or other pieces. I'm guessing that the attachments got separated from the machine and it was on the machine.
Some have a D-shaped slot for the pieces that attach to it and some have circle slot. Some links for instructions. First is the pages for the instruction, then the page for the Attachment Foot and then the pdf for the instructions.
I think I have seen them for a slant shank and maybe a high shank, but yours looks to be for a low shank.
Janey - Neat people never make the exciting discoveries I do.
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