The is a 19th century hand-forged iron quilting clamp, with one of this sort often referred to as a "heart clamp" for the heart shaped finial on the screw. It would have been used for clamping the fabric layers of a quilt (top, batting, backing) to the rails of a frame while working it, sparing the quiltmaking from laborious pinning, etc.

Once a month I go up to midcoast Maine for a show, and once a month, for a few months now, I bring home an old iron heart quilting clamp or two! I've purchased them all from the same good dealer at an antique shop I stop at, and all come from the same old tool collection I am quite sure--the dealer has just chosen to set them out one or two at a time. Last weekend I returned to the scene, and two new ones had been put out since last I visited, both of which I scooped up promptly: I love them as objects--and valentines!-- and they're quite rare in my experience to find. These were typically made in sets of four, and indeed this is the fourth, and I presume final, of a set I've been picking up one by one, selling as I do, not knowing that its mates would emerge! I love the folkiness of the heart on this one, and the relative shortness of the screw makes me think it was forged by the maker for a very specific frame, I'd guess most likely that of his wife! 

4 1/2" t x2 1/4" with screw closed; screw itself 3" long. Very good condition, nice patina, screw turns smoothly all the way closed and all the way open.