p-i-m-a indicate which finger to pluck with (thumb, index, middle and ring, the letters come from the Spanish names), and thus are guitar specific. Similarly the small numbers to the left of the noteheads indicate which left hand finger to use to fret that note.
The diamond shaped notes are natural harmonics. I believe that the way it is notated here is: the note locations indicate the string (e a d g b e low to high) and the numbers above them indicate the fret at which to make the harmonic. Thus the pitches are
- g 19 = d (+ a fifth +1 octave)
- b 19 = f#
- e 19 = b
- g 12 = g (+ an octave)
- b 12 = b
- e 7 = b (+ a fifth +1 octave)
- a 7 = e
where I've indicated where the sounding note lies in relation to the note indicated by the diamond note head. The harmonic produced at the 19th fret is the same as the one generated at the seventh.