National Geographic 88-89 Their ink, screening technology, holes in rotogravure dots

To my eye, National Geographic printed its best prints at 1989 and 1988. I want to ask , who was supplying their ink , what was the exact brand of their inks , which screening technology they were using , is it possible to find the exact screening technology today , and why their printed dots have a hole inside at highlight , mid tone areas ?

Is there a resource to read their inks colorants ?

Long question but I will be very happy if someone shed some light.

Thank you very much, first post here and wish the best to forum members.

Mustafa Umut Sarac
Istanbul