The eight-link is a type of chain connector that used to be readily available at most hardware stores, but seems to have been replaced by maillon-type connectors. My father had one lying in an old tool box. The one in the picture one is designed for half-inch chain, so it is a bit too large to make an ideal ascender, and having the wrong size probably distorts my test results. According to Bob Thrun (in Prusiking), Gerald Wood invented the eight-link knot in 1967 after experimenting with the snap hook knot. (In 2007, William Greenwald sent me two 3/8 links that work better, these are the ones shown in the top photo) Bob's tests indicated that the eight-link knot did not move as freely as an rbs knot - which is one of the better ascender knots. Since eight-links are heavy and no longer common, I suspect that the eight-link knot will remain a historical curiosity.
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Bob Thrun (in Prusiking) reports that William Greenwald invented the Hedden Eight-Link Knot in 1968, after reading about the Eight-Link Knot. Basically, the Hedden Eight-Link Knot is a Hedden Knot with an eight-link inserted to fascilitate loosening the knot. To do so, pull the eight-link down, then push the entire knot upward. The method works well, but the knot is a bit too tall for my taste, and it has a bit of stretch to it. The knot also takes too long to tie - there are numerous simpler alternatives.
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©2008, Gary D. Storrick