36 C.F.R. § 1270.24
When the Archivist may act for a President
If a President specifies restrictions on access to Presidential records under 44 U.S.C. 2204(a), but has not made a designation under § 1270.22 at the time of their death or disability, the Archivist exercises the President's discretion or authority under 44 U.S.C. 2204, except as limited by 44 U.S.C. 2208 and § 1270.48.