The custodian can only indicate the amount of a particular beneficiary's proportionate share of the the total RMD attributable to that particular IRA, but has know way of knowing how much actually needs to be taken from any particular beneficiary. Because of the ability to aggregate IRA RMDs, it's the responsibility of the individual (or in this case, the beneficiaries) to determine what needs to be taken from any particular IRA. This makes information from the custodian is somewhat meaningless. But I think you are fine with whatever you decide to do since the full amount of the decedent's year-of-death RMD was ultimately distributed and a good-faith attempt to show that will have been made.
