Curious_Aardvark: "Use the Search, Luke!"
Dravonk,
Engvall's letter states that he had modified the "traditional" sling, and then he defined what a traditional sling was before he stated what his modifications were.
We had a lengthy discussion of how to get a new record into the Guinness book, sometime soon after I joined Slinging.org. The short version of that discussion: Guinness accepts anything that sounds reasonable and is sufficiently detailed.
Larry Bray set his Guinness record using a sling with a leather pouch (elk, I believe) and Dacron cords, throwing rocks. The record setting rock was about 1.84 ounces, painted red for visibility. I do not know who labelled the objects in this photograph; the lead object is a two-ounce fishing weight, apparently provided for comparison with the size of the rock.
David Engvall set his Guinness record using a sling-like rig consisting of Kevlar cords attached to an aluminium bar with a hook in it, which in turn slipped into a hole in the side of a metal dart. There is a much larger, more detailed image
here
.
Both Bray and Engvall described in detail what constituted a "sling" and what constituted a "rock" and the Guinness people swallowed both sets of definitions (and probably chased them with a pint of Stout).