There can't be a single device added to a coin. The problem is that it would be raised and not incuse. If there were extra letters, then they would all be raised. Often times on a parking lot coin, you will see a piece of sand make an incuse mark on a coin, but the brain tries to identify this as something.
On the zinc cents, when the plating is split and the oxygen reaches the zinc, it will make the surface rise.
I've seen an example of a raised letter added to a coin, but it altered the reverse:
I've seen examples of double struck coins, but the last full strike pretty much cancels out the first strike:
So seeing individual letters is not something that happens on top of a full strike that happens. Sure there are examples of dropped letters, but they can be mirrored or normal, but they are incuse. These are rare, but with a coin in the condition your coin is in, it would not have a premium because of the circulation damage already that has happened to it. Save the best coins, spend the rest when it come to damage on a coin.