I have dug a few in/near beaches, and they are basically unidentifiable except sometimes I can see a mint mark or a date. When I try to ‘polish’ or rub too hard they break like a cracker. I wouldn’t put anything past thoes anomalis planchets
I dug a few silver coins that had this problem, one was a Barber quarter I dug this year...could have been near a fire like Ohiowhiteguy said, or a combination of that plus acid level in the dirt it was buried in all those years.
Because of the Lower Silver Content of War Nickels, (35% Silver)
I find quite a few that are stained badly.
even needing electrolysis to remove the stain.
One thing that just came to mind, usually I see the The Circular pattern
on coins where there are 2 Stuck together.
However, Usually the Circle is the extra clean spot when that Happens.
But because of the Year, what if It had been lost with a Steel War Penny that Rotted away ? perhaps that could explain the circular stain
I remember when you'd find those in your pocket change somewhat frequently. I'm thinking it would be in around the 1980s I started seeing less and less of them and now I see no more. Boy as a kid in the 1960s coin collecting was sure fun.
The effects of unique soil conditions, and possibly heat. I have seen several that have a similar appearance. Silver is silver, but impairments like this I usually put with those that go to the next sale. I typically do not retain ugly silver long.
It just looks to me like the alloy was poorly mixed allowing only that one portion of the coin to oxidize.
I've dug several war nickels but this is the only one that's ever had any oxidation.