A100 was a progression from the previous humanoid models before it, which were called A1. A1's were the original full sized human animatronics created in the 1960s for the Worlds Fair and Disneyland. Carousel of Progress, Lincoln, Pirates etc.
According to Wayne Jackson, they also had A2 (animals), A3 (the Small World dolls) and A5 (dinosaurs). So there were probably some others in the single digits. I'm not sure if they ever had double digit A models in between A1 and A100, or if they just skipped right to A100.