They are common in the printing industry. What it is, is a spec of some type of dirt/metal etc. stuck on the printing plate, that was not getting washed away by the water solution used to clean the plates during printing. If you look at the colour of the circle, you can tell which plate it is actually on. In this case, it is on the magenta plate.
If you've ever scene negatives from old film before the digital age, these negatives are burned into, or embedded onto blank metal plates. Any image burnt onto a plate, whether it is black, yellow, cyan, or magenta attracts the ink to that image on the plate, and sticks to it, and is transferred to the paper, or cardboard. Any area of the plate without any images, gets washed away by the water solution. Occasionally, stuff gets stuck onto the plate, and not washed off. The ink pools around the spec of dust, dirt, metal, or whatever leaving that ring.
The blue line on the back of 1st run Gretzky rookies is really just a scratch across the cyan plate. The obviously changed the plate at some point throughout the run, and the new plate did not have that scratch across it.
I use to be a journeyman printer for the Toronto Sun newspaper.