On the two stamps in Column 4 of each booklet, numbered or unnumbered, I have looked at so far, mint only so far, I have noticed there is a scored line drawn down vertically through the approximate middle of whole two stamps plus the selvedge also.
Only visible when holding the booklet or stamps at an angle to a good light or during datlight, both horizontally and vertically.
Look on the right side of the two dark blue bubbles lined up below the mother otter. I think, myself, that the line is on the backs of the mint stamps also.
This may be from a machine grasping the stamps or pane or sheet, pressing upon them.
Picture showing the approximate position of the scored or pressed-in line: