This just sold at auction for 160,000 Swiss Francs, so I guess with commission just over that in pounds...Sorry no pictures here for now. Very rare and right up my alley but WAAAY out of my affordability range. I have the die module/trial for the reverse gotten some years ago but not nearly as exciting as this one.
My chemistry classes and background beckon, and they say, well that it is an oxidant that chemically further oxidizes the surface for removal of the top layers of the coin. Ammonia does not react with the metal surfaces so would in fact be less aggressive.