Suggestions For Dealing With Toning

Good afternoon all.
I am need of some suggestions for the removal (or reduction) of toning on the stamp in the attached photos. I recently bought the set at auction, the pictures only showed the fronts and the only description was 'm/m'. So when they arrived I was a little disappointed with just this one stamp.
Any suggestions welcomed, but I don't want to do anything drastic that might damage the stamp further. Thanks in advance.
Best regards
DavidR

Chloramine trihydrate is a conservation-grade mild bleaching agent used to remove stains on paper in a 2% solution. Otherwise hydrogen peroxide or troclosene sodium (sterilising tablets), both of which might damage the stamp.

That's disappointing, no doubt.

If it was mine, I would (a) immediately get it away from the others, and then (b) replace it (I see nice copies with lovely centering online for under a dollar), and (c) play around with a few cleaning methods.

Hydrogen peroxide, and/or a very dilute bleach bath, and/or a quite warm bath with a gentle detergent, followed by a cool rinse. Regardless of the cleaning results, I would not reincorporate it with the balance of the set. The cleaning methods would only be for my own experimentation.

If the set was on a dealer card or in a glassine, I'd move the others to a new home right away.