I thought of that. There's no fourth in the melody, but there is a flat
9, sharp 9, and sharp 5, and in one place a natural 6 (which would imply
a diminished scale, but with a sharp 5). (I'm at work, and don't have
the lead sheet with me right now, so I'm going from memory.)
Maybe it's an alt chord with no third, and a sharp 4 sus--i.e., G in the
bass, voiced upwards C#, D#, F, A# (in other words, C#69/G)?
As I said, I can't tell the difference between the SD chord and a
regular alt chord, either from the lead sheet or by listening.
I guess it's a moot point--if it sounds like an alt chord, and resolves
like an alt chord, it's probably an alt chord. But the notation threw me
off, and I thought it might affect the bass or guitar voicings (I'm a
horn player).
None of the other tunes on the album have "SD" chords. For all I know,
it's a copyist error, and I'm chasing a red herring. I just thought
someone might have seen this before.
HP