Ok,
No responses, so I looked at the rear stock again. I was able to fit an allen wrench into the swivel stud hole and unscrewed the metal insert. Then it became obvious that it was a quick-detach swivel cup. I was able to use a gile sling coupled with a nordic components QD attachment to the barrel/mag tube clamp.
And also figured out that if you want an old-fashioned swivel stud, the above part number C5E053 also screws into the buttstock and works well.