Those arrows are identical to my old graphlex arrows. The graphlex are marked the same way, like 17-6, 17-8 and 18-8. They made several sizes. I have used these for years, and killed my buck this year with a 17-8. They are fiberglass with strips of graphite embedded in the arrow. The 17-6 works well with 50-55 pound bows, the 17-8 will work well with 55-65# bows. I used to use the heavy 18-8 shafts with my old 75# compound bow. They are fairly heavy shafts, and tough as well.