The white strip is a piece of fibre, cotton based with bio-organic adhesives that are water soluble. Unfortunately an awful lot of split paper from the 17-18th century got taped together with scotch tape during the 20th century, and guess what?
Scotch tape eventually breaks down, the adhesive chemicals in the glues absorb into the paper, making clear spots and ruining the eye appeal.
If something is professionally conserved, they take the approach like using natural fibre based materiel with organic adhesives that are water soluble so that it can be taken off at a later time if desired without affecting the historic document, currency etc.
I have an old book from Scotland that was printed in 1806 that I put part of the cover back on using a paste made with a small amount of water and plain old baker's flour. Baker's flour combined with water makes a decent binding agent for books etc. and is easily removable of course.