Have you by any chance consulted Robert P. Elmer's chapter on Bowstrings
found on pp. 323 through 351 of "Target Archery
", Alfred A. Knopf, New York, 1946? He researched the matter quite thoroughly and quotes many of the leading archery books/authors on the subject and may well have the answer you seek. He distinquishs between flax and hemp and also notes the use of silk. He also mentions Duff although tangently on pages 328 and 329. Hope this might be of help!