Hi WTF,
The original D-9 is hollow ground, and BG-42.
The D-9b is flat ground, ATS-34. Other small details such as a blacker tiger stripe on the original D-9. Some of the grinds of the "b" were just slightly less than perfect, (hard to see unless I was told) so Duane let them go for less money. I think the "b's" were only a small batch.
Thanks,
Scott