Great find and good eye too! Finding coins like this is when it really pays to save the pre-'60 nickels. I need to go through my pre-'60 nickels to check for any errors/varieties too.
At the mint, after the die is made the mint mark is stamped into the die. When the employee is told to change the mint mark, they attempt to erase the old before stamping on the new mint mark. If the old is not completely erased, the above is what is found.
That is awesome! I've never checked for this error, but knew it existed. I have something like $200 FV in pre-1960 nickels (don't judge me!! ), and I might have to look for that error
Thanks for sharing,
HH
Buff