Hans,
There is no doubt that they are factory installed. I'm in the process of cleaning the underside of my current project car 280 SL #1022 and
the anchors are mounted after the PVC protection layer was put on, but before the application of the brownish protective wax.
Actually on this car they are not in use, the manual three point Kangol belts found in this car uses the other factory less crudely :-) fitted anchor points.
I am guessing that these parts were installed to accommodate other seat belt types required by some state or country law or some owners special wishes to have two point lap belts only.
No mention in the parts book about any such added plate anchoring points.
/Hans in Sweden
I haven't checked the parts book, but they are listed in the EPC, and they are still available according the MB Classic Parts Search website.
Attached a few pictures of showing the anchors from the outside and the inside.
Hans