The Colt Sporter Leather Sling was provided as a factory accessory and was also sold separately. You can see a few more details and images here: https://thecoltar15resource.com/colt-sporter-leather-sling-pn-ca10054/
The Colt AR-15 Resource
"A site dedicated to the history of the Colt AR-15 rifles, carbines and associated items"
The Colt AR-15 Resource 