1. R8BC004 is the latest one but you can use any of them. The ones that are branded are called T_Mobile, Vodafone..etc. Pretty self-explanatory and also explained in the tutorial. Unbranded=Generic
2. Unfortunately there isn't, but you can look it up here:
Downloads - k770custpack - Top Sony Forums
To anticipate your next question: RxA in the name of the custpacks stands for the revision number, don't get confused about it.