[sowbug]

The article mentions the "registration" process. Those multicolored gunsight-like symbols you sometimes see on the edges of printed things are called registration marks. They help the operator quickly judge whether the plates are aligned. Slightly different from the pins and stops the article mentions; those are to align the screens, while the marks are to see whether the result is aligned.

This is all familiar to people above a certain age. Younger folks who grew up without Sunday color comics might still be able to find a slightly blurry plus sign on the bottom of a paper milk carton.