I purchased/exercised my ISOs in 2017, in 2018 the company was bought out and my stocks were sold. I calculated the cost basis when entering my income from this sell in my 2018 tax return. In the terms of the sale, previous shareholders would get additional payouts/distributions based on how many stocks they owned at the time of the sale. I didn't own (or sold) any of this stock in 2019 but I received the additional payouts. My guess is the cost basis is 0 because I already filed it in 2018 when the actual sell happened. Any insight is much appreciated! I hope that helps clear things up.
