"No idea how they got there finds" are the most fun of all...I bet an Irish Immigrant girl brought it here as a necklace and lost it. This person probably came here during the famine of 1849, and settled in Ma. The necklace was given to her as a love token because the giver was a poor Irishman who had a penny but did not have the price of a proper locket with his picture inside. And then later on when the sorely missed loved one from Ireland finally arrived, the wearer of the necklace ran out into the street and hugged him so hard the chain broke and the coin laid there until you found it!
Yeah, I know, I got way too much of an imagination...interesting find.