It is a clamp for when you are stitching leather so you are hands free...when stitching leather you are working two needles at each end of the length of yarn. Without the clamp it is a bit of a mission and it helps to keep the tension of the stitching more consistent. The bolt tightened with a wingnut keeps the pressure on the top...the leather on the clamp helps to avoid bruising the leather you are working.
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