These are leftover layout marks from the transfers used to lay out the plates. They're usually a bit big before being erased. The marks are used to make sure the plates are laid out correctly and that each color matches the others.
They're not always consistent, so any particular color for any particlular front might have them or not. They'll eventually be a help figuring out possible sheet configurations, since it's possible to identify different positions.
Here's the top center of Batch showing Black,pink,and blue on one, and black and a diffeent blue mark on another.
There are also a few odd remnants found on some corners. Batch again, the same two cards. One with Light blue having a stray line to the right with darker blue showing small one towards the top. The other has a light blue or maybe gray mark pointing up, that the other doesn't have.
I've been using them to get some idea of how many times a particular card might have been on a sheet. Mostly looking at the rarities, Wagner Plank and Magie which were all probably only on one sheet. So if there are say 6-8 different Magies, that makes me fairly confident that each card was there 8 times. (Approximately, I think the layouts were a bit more complicated than that.)
And yes, there are 6-8 very slightly different Magies, all with back differences that are consistent, so if the front has a particular pattern of alignment marks, the back will always have the same flaws.
Steve B