This is a brand new (old stock) US military surplus Black Silent Sling, or Small Arms Sling. These are used on the M16 series rifle and can also be used on many other rifles as well. These slings can also be cut to a shorter length if needed to customize for your use case. They are also great as a general purpose sling for carrying heavy loads such as ammo cans. These straps are 1.25" wide and approximately 50" long.
NSN: 1005-01-216-4510