To add: Sometimes slings were made on the commercial market and purchased by guys in the service on board Tanks. The reason is tanks had internal issue the M3A1 SMG, whcih used the carbine sling.
Many times as late as the 1980's a guy would purchase a commercial sling because his sling failed. After time, when items were turned in, fake slings ended up being in batches of Government Surplus Release.
I have had several fakes slings in original batches of GI slings which I won on bid.