F maj7#5, as the Bass Takes It
The grip Guitarrón uses for F maj7#5, and the job it is doing while the chord sounds.
F maj7#5 spells F – A – C# – E. Which of those tones reach the air, and in what register, is decided by the section holding them — so the grip below is the answer for one instrument in one section, not a universal fingering.
This instrument is the whole bass section by itself — six heavy strings on a fretless neck, roots and fifths on the strong beats and an approach into the cadence, nearly every note doubled at the octave. Almost none of a chord is playable down here and almost none of it is wanted — the ensemble hears the bottom note as the chord, so what matters below is which tone of it gets chosen and which octave it is doubled in. There is nobody to play it in unison with, which is the one section that answers to itself.
This neck carries no hardware to stop against either, so the diagrams below are 12-TET reference positions on the A–D–G–C–E–A fingerboard rather than something you can feel for. Treat a charted grip as a map of where the pitches are; the working technique is a pair of strings pulled together an octave apart.