I'm not sure how valuable they are either, but also brought back memories.
My mother quilted on this type of frame. She inserted bolts or large nails in the corners holes to assemble the frame. Mounted the quilt sandwich with a basting stitch looped around the rails,and then as she quilted a row (as far as she could reach, she would undo the basting to that point and roll the quilt and insert the bolts again to keep it tight.
It was hung from the ceilingm when she wasn't quilting she wound the lines around the ends of the frame to get it up and out of the way.