Did you acquire the stamp in the early 1920s?
If not, what causes you to write:
No fluids has ever been used that I know of.
The photo is not clear. Maybe another stamp or paper got stuck to it at some time. However, it looks like the stamp has been damaged by some liquid, possibly a solvent.
Edit: Maybe Don, our moderator, can say whether acidity of the preceding album page can have caused this.