I've never seen a leather sling for an SKS. They've all been canvas, some with leather ends but never all leather. That's not to say some iteration may have had one. but that one looks a bit too "sophisticated" for an SKS.
I think when those slings were imported they were sold as G3 slings of some type, the buckle looks dead on those slings as does the thinner then normal width. They do work with alot of firearms though and being leather and brass look the part on many of them