Further clarification, informed by (gasp!) research, not just guessing:
"United States National Museum" was not synonymous with either
"Smithsonian Institution" or "National Museum of Natural History," as I
indicated in an earlier post.
  The USNM was the museum component of the Smithsonian Institution
(which still has administrative units that are not museums); the Museum
of Natural History was PART of the USNM, not synonymous with it. In
fact, the Museum of Natural History was not created as an administrative
entity until 1957 (the BUILDING obviously is much older), at which time
the Museum of History and Technology was also created as an
administrative entity under the United States National Museum.
  The BUILDING, Museum of History and Technology, did not open until
1964 (and the name was later changed to National Museum of American
History, for better or for worse). The United States National Museum as
an administrative entity "ceased to exist" in 1967, although the
Smithsonian Archives' USNM records go to 1975. This info is available in
"Guide to Smithsonian Archives" (1983), p. 22, cited in the "Chronology"
database in the online catalog, SIRIS. BTW, I still have leftover
"United States National Museum" stationery. --David Haberstich