Well Ben, I enjoy the selfbow thing too. That bow is a sweet shooter, going to make someone's teenage granddaughter happy I hope.
Mike, that's just temporary lacing. I think it's the stuff you buy to go with a stitch-awl. I have a big roll of it. When I put them back on after dying and finishing, I use a dark brown, waxed, linen thread. Looks like this:

Sinew backed, osage crossbow Clint? Suhwheet!