Josh,
Thanks for the reference to Keith's article. Indeed, the construction does appear consistent with known double rifles produced even earlier than this one.

Given that you could order a Lefever just about any way you wanted it, especially the high grades, somebody who wanted an AA with Whitworth barrels would probably not be denied, although I am not aware that they were advertised at that time.

Great-Great Grandson of D.M. Lefever
www.lefevercollectors.com