I always was of the opinion that they were an eye focus instrument, not a peep sight.
To be used with the rifles existing front and rear(bbl) sights.
The idea makes it possible for the shooter to have the target, front and rear sight all in focus at the same time.
Same idea as the Merit Disc that many Bullseye Pistol shooters used back when that was popular. Especially the older shooters that had
problems focusing on the front sight,,,or either sight actually.
They still make these and they have a nice explanation of the principle in the ad.
I think the Merit had a non magnifying 'lens' in them, but not really sure now.
https://www.theshootersbox.com/merit-optical-attachment_p_13939.html