I thank both of you. The cord I referred to was found in the quiver and it was assumed that it might be a bowstring by the writer, it turns out that it was unsuitable for this purpose.
No problem. Some further searching turned up an article in the November 2011 National Geographic that contains a number of photos including a few of the knotted items. The article starts on page 120.
One of them shows a coiled/tangled length of rope which the caption suggests is "possibly his bowstring" -- very likely the subject of your query. The end knot is visible and may in fact be the Overhand Bend (#1410) noted earlier. Given the thickness of the material entering and exiting the knot, I'd say that seems quite likely. If it is a single-strand terminal knot and I had to guess, I'd say it may be a right-handed (Z) Double Overhand/Blood Knot (#516). However the orientation in the photo is not ideal for identification. Given the (incredible) lack of deterioration I'm sure one could immediately identify the knot if examined in person -- or at least provided with a set of carefully made photographs intended for that purpose.
There is also a whole-page photo of the mesh-like/sandal portion of his shoe that really shows some of the fine ropework. There appear to be some nice little tricks done with the plying to branch subsidiary cords from a main one. If he made these himself, he was skilled indeed. The knife sheath is also very clean in its design and has the look of something the maker had refined over many iterations.
If this issue of NG is not available at your local library, there are several copies currently listed at reasonable prices on eBay. The photos are quite nice (it is NG after all) and don't appear to be duplicates of those available on the site Wed posted above.