Natalie Dessay performs Bach's very beautiful Aria "Höchster, mache deine Güte" (A minor) in these two videos(only the sound and the score) (with video of performance)
The score is available at IMSLP: http://imslp.org/wiki/Jauchzet_Gott_in_allen_Landen,_BWV_51_(Bach,_Johann_Sebastian)
The score is originally for cello (basso continuo) and voice: however, the performers also use an organ, which is not listed among the instruments, with particular chords: Am, Dm7, G7, C, F, Bdim, E7, Am, ... (B stands for B natural)
Clearly those numbers don't correspond to fingering because the number of fingers is limited by 5. Those number also don't correspond to strings for the same reason. The number 7 for the sept-chords could be a bit confusing because the 5-th number seven in the beginning (B-C-D) sounds like a diminished chord Bdim, but then the sharp (E-D-E) is logical because we use the dominant sept-chord with G#. Anyway, I can't extend this logic because I don't understand the meaning of "6-4-3", "6", "6-5", just "#", "9" because I could try to interpret them in terms of some jazz chords but this doesn't fit into Bach's barocco style, and also doesn't fit to what I hear on the recordings.
Do these numbers have particular meaning? If they correspond to chords, is there some particular algorithm to convert these numbers into chords? Or maybe this organ part is just a custom arrangement of the performers?