I've got a couple of slings that look just like that one in the OP's pic.
A now deceased very good friend of mine, made them up on a regular basis, for his personal use, as well as gifts.
He was very good at sewing everything from canvas to leather and everything in between.
Made some of his own clothes as well.
I believe that sling was built by Rowan Grassi, when he lived on our range as the caretaker for over 20 years.
He described it as an island of sanity.
Most of the later slings he made were made from rolls of material that was very similar to that used for seat belts.
He made them up in rainbow, brown, blue, black and a few in pink for the ladies in his life.
They are actually quite good, don't snag, wear or stretch/shrink.
He loved to shoot firearms and cameras, even made up his own lenses for his century old cameras.
Great conversationalist and friend. I miss him.