My grandpa started a custodial account for me through TD Ameritrade, I did not know anything about the stocks he purchased. I have now taken ownership of the account and have received a 1099-R. I could fill out the long and short term transactions just fine, but there is a section for undetermined transactions. I do not know when or for how much my grandpa purchased these and he has now passed. How do I go about finding the information needed to finish the form?