I'm helping someone who just started taking RMDs from a traditional IRA. They have no idea of their basis. Contributions would have been made in the 1970s & 80s and their tax records from that period are at best fragmentary (although in at least two years it included letters from the IRS questioning the deductibility of their IRA contributions but no indication as to how the matter was resolved).
The IRA custodian doesn't have online records going back that far and they probably wouldn't know basis anyway.
Can the IRS tell us when the last 8606 was filed and what was on it (after any IRS changes)?
Is assuming $0 basis their only option at this point?