Anyone got any idea what these single brackets on notes mean in this guitar music?
I've just gone through a number of baroque books and looked at buying the Neumann book mentioned above (Not going to happen at that price though). I finally found the answer though at http://baroqueguitar.homestead.com/The_Baroque_Guitar.htm which states that the bracket means a mordent. I hope this is helpful to anyone else who has been puzzled by this.
It could be some type of ornament from the baroque or renaissance period. It would help to identify the piece, the composer, and the edition of the music the example came from. The sign is certainly included in the recent comprehensive standard for music fonts - see glpyh U+E572 here: https://w3c.github.io/smufl/gitbook/tables/other-baroque-ornaments.html
A comprehensive reference (which I don't have access to right now) is
Neumann, Frederick. Ornamentation in Baroque and Post-Baroque Music. Princeton, NJ: Princeton University Press, 1978.