They were not "original" on the early snipers, but many were replaced during refitting and refurbish at the request of the unit armourers.... Or, as I suspect during sniper school or training...
Of all the rifles I've seen with sniper sling swivels on them, I have yet to see one being used for anything other than a decoration...they look cool, I love them, but did the snipers actually hook their sling in them? Even my Cno7 has one.