My Remington 870 Marine Magnum has sling studs installed...
it was bought used (by my father first) and had them then...
so, it MIGHT have been added.
BUT!!!
I have seen other Marine Magnums with the same studs, located in the exact same place, so my thoughts on it are that it came from Remington this way... (indication #2 is that the mag extension clamp assembly includes an intergal foreward mounted stud, so the rear one is likely factory also...)
you might inquire (at a gunstore, or with an E-mail to Remington) about this...