To my knowledge the number depends on what the rival is. e.g. the 306 is because this is a Prototype machine. So fair to assume that 4xx is Military grade, 5xx is FNIX and 6xx is Apoc + Reaper (Cant remember Reaper numbers so i assume its 6xx). I can also provide the 3 names that every machine gets as a rival, or at least all the ones I remember, if that is also something you want to be stated.
You can try it yourself if you wish.
It seems a very boring research to do.
There are many types of names and most of them are similar.
If the rivals had a different appearance and weapons loadout I wouldnt mind doing it, but that is not the case, there are only 2 things that change :
- Extra Health
- Having a title