There is no such thing as "standard M16" sling. The sling called for for both M16 rifles and XM177 series is the M1 sling. Please remember that the M1 sling has nothing to do with M1 Garand, it just happens to be the military designation (nomenclature) for that model sling. Cotton canvas from WW2 up until 1966,followed by the 1st contract for the nylon "seatbelt" sling in 1969.
Also shown in the XM177E2 manual, an alternative for that carbine IS the BAR sling. (if you happen to have a BAR manual and the XM177E2 manual), you will note the same NSN number.
For the record, I recently learned that there were NO cotton canvas slings contracted after 1966 by the US military . I had posted a Q asking if anyone had a 67 or 68 dated cotton canvas sling and no one had any. Now I know why.