Acting or Under Secretary of State? 

Cordell Hull was Secretary of State from 1933 to 1944, Sumner Welles was Under Secretary of State from 1937 to 1943.  In 1940 Cordell Hull was Secretary of State (nominated, confirmed and sworn in). In 1940 Sumner Welles had also been nominated, confirmed and sworn in as Under Secretary of State. In the normal course of events, there is only an "acting Secretary" in the absence of a confirmed nominee (e.g. - L. Patrick Gray as acting Director of the FBI after the death of J. Edgar Hoover).  The Under Secretary of State may act in lieu of the Secretary (due to vacation, official travel, etc) but is not normally referred to as "acting Secretary" anymore than a Lieutenant Governor signing a bill when the Governor is unavailable is the "acting Governor," he/she is arguably "acting for the Governor" unless there has been some official transfer of authority (as for instance when the person is undergoing surgery - usually that is referred to as "temporary acting..." - if Sumner Welles was Acting Secretary of State there should be some explanation as to why (e.g. "in the absence of Cordell Hull on diplomatic mission to....")