"Just had another thought - maybe the '-9' part of the name is another way of describing a flat 9th?"

That's precisely it, Bill. It is a flatted 9th. If you played a D it would be a C7add9 chord. A 7-9 chord is really a diminished chord in the way that it sounds and the way it is used.

I agree with you John. But I am still groping for the correct way to play a bass line with a 7-9 chord. Are you saying that for a C7-9 chord, a "C" in the bass line is correct instead of a C#? The inversion that is used for the chord would determine what bass notes were chosen by an arranger keyboard for the APC part. I'm not all that sure that an arranger can get it right for the bass.