In addition to the sibling poster’s comment, publications had data on what parts of town and stores sold their publications at newsstands and the names and addresses of subscribers.

Before you could get free publications like MacWeek, you had to fill out demographic data to qualify.

Nielsen ratings would disagree with this.

I think that's a disingenuous reading.

Nielsen ratings are for small subsets of users who explicitly opt in to tracking. That's not the same as "uniquely identify all end users of my website" it's like paying user testers.

Nielsen tried it’s best to get a demographically representative sample of households. It wasn’t purely a self selected sample. I doubt people were incentivized by the $5 they put in envelopes.

I didn't say self selected, I said opt in.