

it's environmental damage, what made it different happened after the coin was minted. That means it's not a mint error and without a witness, it's tough to know with certainty exactly what happened to it. If you see an extra design on it, there's a chance something was placed on it and via various pressures could have imparted the extra design.
In answer to your other question, you can add multiple coins to a single thread, but I don't recommend it. If you limit it to one coin per thread, each coin receives more attention.