Did you find a letter or number that is the same size and detail? I really can't tell anything from the photos. I will just say it's a well circulated dime.
Like @Pickin and Grinin said. Also just like @potty dollar 1878 would say, Just a well circulated and damaged dime. Ooooh, you are potty dollar 1878!
I don't think its a dropped letter. Too indistinct on the shape. Many times, a heavily damaged coin has other anomalies that look like errors, but are only PMD. Even if it is a dropped letter, you're going to have a hard time convincing others that it's one. Too much damage and not enough detail, IMO
