I was fascinated a few years ago by a guitar made by Vicente Arias 1900 because of the quality of the work as well as the fact that it had a double back.  This demonstrated once again how today’s “innovations” in guitar-building are very often not innovations at all.  Now my colleague Aarón García has brought another example to my attention of a double back but this one from the 18th century.  Here is what he says to me:

This is very interesting for a few reasons, for example the absolute indifference of the Spanish researchers about Portugese chordophones…   Another curiosity is that this instrument has a graft under the top in the position of the bridge. The string length includes compensation by gluing the bridge on further back.  So many innovations in one instrument!

Here is the reference:

MICHEL, Andreas; NEUMANN, Philipp, Gitarren. 17. bis 19. Jahrhundert. Musikintrumenten-Museum der Universität Leipzig. (Catálogo). Leipzig: Musikintrumenten-Museum der Universität Leipzig, 2015, p. 94-98.