Based on the sling used in the above raid during the VietNam War, this is not much more than a nylon GP strap with some added 550-cord loops.
While a basic design, being a lot wider than the issued sling of the time it does a great job of taking the weight of the carbine due to this wider strap, plus, as a bonus it keeps the gun the correct way up for faster use!
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More details to follow