For my 1979-80 Trek 730 frame, the last number designates the component group. Specifically, the 730 frame was available with component group 006 (Shimano 600EX), 007 (Campy GS), or 008 (Campy NR). I built it as a 738. The same component options were available for the 930-series frame of that year, and slightly different component options were available for the 510-, 710-, and 910-series frames. That's my understanding of the early system, anyway.