Since the previous discussion about the 1935 $50 note in question has shifted and gone off-topic, I will make my comments about the actual note here.
The original stain looks like it could be from tape, and tape stains cause irreversible damage over time as the residue does not just sit on the surface of the note, it penetrates into the note, causing discolouration and changing the texture of the paper, making it more brittle. Even delicate handling of a note with an old tape stain could end up causing the paper to break and literally crumble away, leaving a note with a small portion missing.
HOWEVER, I am leaning away from believing the stain to this $50 note is caused by tape. The repair appears to have eliminated the stain, meaning the paper was not damaged. A few years ago, I bought a nice early 20th century mid-grade Chartered bank note that had been hinged along the left margin to the inside of a book. The seller literally removed the page from the book and sent me the note still hinged. The adhesive looked brown like a sap, not like a glue. So I moistened the paper along the hinge and the note literally separated from the page with no damage and no stain because the sap was water-soluble. The only difference made to the note was a slight ripple along the edge where the water contacted the note. Since the 1935 $50 note is only slightly younger than my previous-owned chartered note, I wonder if the stain on it was a water-soluble adhesive that lifted right off without the use of soap or chemical agents. Without seeing the note in hand, I can only guess.
I sold my chartered note without disclosing that it had been hinged to a page in an old book. Since the sap dissolved and left no residue, I don't think I was wrong to not disclose the note's provenance of being "previously hinged/mounted". And it certainly was not cleaned because water came into contact with the note only along the hinge (which was quite thin since the sap had been applied sparingly). The other 99% of the note was in the same condition as it had been inside the book.
The owner of the 1935 $50 note in question professes that the note is still "original", and that's obviously a mistake. However, the note does not exhibit originality in either picture. It has a flat, lifeless look both before and after. So I can't say much of anything about how the stain's removal changes the note's desirability except to say that the note has become "toxic" due to this forum since both the before and after pictures exist. The cleaning is almost a moot point. The owner is quite understandably embarrassed!
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