One further note:

It's an unanswered question whether the "Open D" tuning originated with the 1888 "Sebastopol" or existed before then.

Anyone have evidence of use of open D guitar tunings BEFORE 1888?

The same thing can be said with respect to "Open G" and "Spanish Fandango." I know of no traces of the American guitar/banjo pieces called "Spanish Fandango" (there are at least two) much earlier than the 1920s.

It's true that fandangos as a dance genre go back well into the 19th century. However, our native "Spanish Fandangos" bear no resemblance to genuine Spanish fandangos. They are southern American instrumentals, whose source I don't know.

(In fact, I have known my version of "Spanish Fandango" so long I have no idea where I learned it -- talk about the mists of history.)

It would be worthwhile investigating the earliest history of both these widely popular open tunings, because they provided, among other things, the foundations for country and blues slide guitar as well as open fingerpicked styles. Off the top of my head here's a capsule outline for research:

You'd have to take into account the Portuguese cowboys who went to work in Hawaii punching cows, and brought open guitar tunings with them, which became the roots of:

1, The phenomenally popular "Hawaiian guitar" slide style beginning in the early twentieth century and fostered by such widely popular duos as Ferera and Francini. (Chinese restaurants, whose cuisine also developed in Hawaii I believe, commonly featured Hawaiian slide guitar musicians and spread the music that way -- it was also a hit I think at one of the World Expositions at the start of the 20th century.)

2. The lovely fingerpicking equivalent known as "slack key," enormously influential especially after 1940, based on open tunings. Since "Spanish Fandango" strongly resembles slack key playing, it may have originated there.

In Mississippi W.C. Handy reported hearing a slide guitar bluesman playing in a railroad station -- I think in 1903 (could be off by a year or two) -- the earliest such I know of. (Some have speculated it could have been a young Charlie Patton.) Slide blues of course used both open D and open G, and guitar bluesmen not only used the slide, but picked in open tunings too. (Some slide bluesmen began on a one-string instrument using an unraveled broomstick wire, but they soon graduated to the guitar.)

So if you go back far enough, you should find a ferment of activity using open tunings, perhaps sometime in the 1890s. As for anything before 1888, it's certainly a possibility that deserves a closer look.

But