Years ago (late 70's/early 80's) I read a science fiction story which I think was a novel, though I'm not sure. In the story, there was a secret group that had developed technology for travelling between parallel universes. In the multi-verse of the story, there were a great many time lines with alternate US histories and in particular differences in who signed historic documents - I think the Declaration of Independence was the main one, though it could have been the constitution. If I recall the story correctly, there were collectors who trafficked in rare alternate versions of these documents (our version was quite commonplace).
I remember one scene (the first chapter?) in which a historian who specialized in the document in question was visiting the National Archives and reading through it and fainted when he got to the signatures. Someone had substituted a rare version of the document for the actual one from our universe.
Can someone identify the title and author of this story?