I found B's on note stems in sheet music of the 3rd movement of the Libra Sonatine, "Fuoco", by Roland Dyens, which is for guitar: What does that B on a note stem mean in this guitar music? Those notes with B's are not played entirely cleanly, and part of me suspects that the B has to do with barre, but I still cannot figure out what the B is entirely for (unlike Z's on note stems earlier in the music, which are for mordents).
The sheet music scrolls by with the music in the following video, and notes with B's are played starting at 13:49 in(the video should start at 13:49).
Even after taking a good hard look at Roland Dyens playing "Fuoco" in(he starts playing notes with B's at 3:14, and the video should start there), I remain confused - he didn't even seem to play the notes with B's in a consistent fashion, especially not the ones in the last measure of the piece.
For what it's worth, my Google searches for what this symbol means have produced only irrelevant results, and I couldn't find any guide on what those B's mean earlier in the sheet music.
(When I made an Liszt's-Paganini-Etudes-style enhanced transcription of "Fuoco" for piano, I ended up treating the notes with B's as if the B's weren't there. To this day, I am not completely satisfied with how I treated those notes.)