First thing is we know that there is nothing in the minting process that would cause this kind of damage. Even if the damage was caused at the mint it would have occured after the coin was struck and therefor it would still be considered "post mint".
Now to explain the most likely way this damage occured. I believe this was caused by the same machine that causes the damage to the date on so many coins, the crimping fingers on a coin rolling machine. The bottom of the letters are approximately the same distance from the rim as the last digit of the date that gets damaged so often from the roll crimper.
Richard