Why Is There A Line Pattern On The Back Of This 1898 France Stamp?

There is a strange line pattern imprinted on the back side of this 5 c.,French ,Sage stamp.
If these are traces of the cover on which it was mounted why is the form of the lines uneven ?

This isn't the 1898 issue, which is pale/yellow green.

Paper looks normal to my

Paper looks normal to me for 120+ year old USED stamp.

In my opinion, spending time paper flyspeccing decades old used stamps seems like tilting at windmills. Used stamps have an unknown history by definition; and paper is highly prone to damage, aging, and other potential altering situations.
For example, many people 'press dry' their soaked stamps, who know what could have been pressed into the back of the stamp shown above. Try taking the stamp above, soak it, put it between two paper towels, then stack some books on it to dry.
Don

Thank you all for participating in the discussion.
I haven't tough about this. The paper towels explanation sounds very logical to me . The pattern may have imprinted it self on the paper when it was placed on it for drying. Of course it is impossible for me to know how it's previous owners stored it before l have bought it.