I’m trying to understand the letters and punctuation in an old piece of music (from around 1910). It’s a piece for ukulele and I’m assuming it’s a fingerpicking pattern, however it doesn’t use letters I would normally see. Does anyone have any understanding of them?
This is called the "tonic sol–fa" system, and it's a variant system of movable-do solfège that also incorporates rhythm and meter.
The piece is in B♭ major, so the text indicates "do sol la sol" by abbreviating to just the first letter of each syllable. Symbols on either side of the letters indicate metrical strength within the measure (on the left) and register (on the right).
For more on this system, see What exactly is the "tonic sol–fa" system, and how is it different from solfège? Note that the metrical symbols in your excerpt are slightly different from the metrical symbols shown in this answer.
