Some refer to these as "gradual progression of under inking"
Thai, The reason I wanted to see the front is that IF a note is altered there would be some sort discoloration / strange things taking place on the opposite side of the inking issue.
Below is pic of one of my purchases which is under inked. On US notes this would have gone on for about 7 to 10 notes before the ink reservoir filled back up. (the reflection is my shadow as the note is in a pcgs holder).

Nice find / Pick up

