With my NRA "service rifles", I can clearly see vertical stringing in the prone position.
As your face slide up and down the stock, the point of impact on paper goes up and down. I think it goes, move up, the point of impact moves up.
People do not want to acknowledge that the greatest error source is the shooter, and the greatest source of shooter error is inconsistant position. Call it a poor "stock weld".
I saw this today shooting standing with my M1a. As I moved closer to the sight I had to add elevation. I shot a 95-2x standing. That was my best string, my position stunk today.
You notice vertical stringing when your groups go up and down, but not sideways. With a target rifle, it is easy to figure out that the cause is poor position.
When I have rifles that vertically strung, in all but one case, it has always been due to poor bedding. Seldom will poor bedding only show up in vertical stringing, you will also have variations side to side, but I think the biggest deviation is vertical.
I have one Ruger #1 in 30-06. It vertically strings everything in a perfect line. The line is about 2" high. I have played around with tension screws, free floating, pressure bedding. The thing vertically strings.