I don't know if you had already noticed it: on behind of the banknotes

and

there are two small lines that they form a 90° angle, placed in a precise point that is the same one for every denonimation and different between dissimilar denominations. On notes of some States they are good marked, in others nearly invisible.
On

they're immediately over of the small map of the Carribbean French islands, grey-colored, while on

they are immediately to right of the first pylon base of the acqueduct, in the middle of the iridescent strip, yellow-colored.
I suppose that there are in other colors and positions also on the other denominations, but I don't see them.
European soul, European pride.