afaik the extra pin of the opteron is just to make a64s and opterons not plattform compatible.
939= desktop unreg memory
940= server reg memory

with reg memory they can run more memory at a higher speed, but the latest memory controllers run fine with a lot of ram at high speeds, thats why you can run opterons with unreg memory (if the bios/board supports it)

and afaik the only difference between the fx and the opteron is that the opteron (usually) uses reg memory wich means a performence drop in some situations.

and from my understanding its like vapor described.
a64/fx/1xx = 1
2xx=2
4xx/8xx=3

what "system communciation" do you mean does the opteron have to handle?