Near as I can tell, either this coin was lying next to something that caused a strange chemical reaction . Hence, the corrosion.
*OR*... as I strongly suspect...something funky was being attempted. The corrosion itself seems understandable enough. But, for it to just co-ink-il-dinkily....be right over the mint mark. I suspect somebody was using acid to remove the mint mark. Maybe an attempt to create a Henning?
In any regard, a truly interesting find in my book. Boy! If it could just talk!
It looks like it might be 1943 over 1942 nickel, based on the barb on the bottom of the three, may just be the lighting or corrosion though. I have found war nickels with that type of corrosion, normally it is due to the large amount of copper in the silver alloy creating impurities within the metal, and then corroding. Remember, war nickels are only 35% silver, therefore they will not hold up as well as a 90% silver coin.