"sparky", get a copy of "Steinschloss-Jagerbuchsen" (Jim Chambers has them) and on page 550 you'll see an awesomely beautiful jaeger made by Ron Scott equipped with a sling. Assume Ron either made the sling or commissioned it to be made, but my German language ability isn't good enough to figure it out-the book is written in German!
Tim: quickly looked through my copy of above book, and while many, many of the jagers pictured have sling swivels, no original slings are shown that I could see. On pages 497 and 541 there are a couple of photos showing a little, but not much is given to go on. (Will be in contact with you soon about the alligator tooth powder measurer.)