Found the answer, 3# per thread is correct http://www.ehow.com/about_6131194_fi...e-history.html
Linen fishing line was popular with anglers until after World War II. It was the line of choice for big-game fish anglers because it was strong. The strength of linen fishing line depended on how many linen threads were braided together to form the line. All lines were made in multiples of three. At one time linen was available in up to 72 threads, which was considered too heavy for sport fishing. The International Game Fishing Association recognized linen line of up to 39 threads, which was 117-pound test.
Like silk, linen also had to be specially cared for to prevent the breaking down of the natural fibers by bacteria, mold and ultraviolet light.
117 divided by 39 = 3