Listen to Jerry- The man KNOWS FALs!
Uhmmm-not really.That is my understanding as well, that the SA-58 is completely new whereas the STG-58 is a typical mix of both surp and new components.
They both are a hodgepodge of new and used parts.
The STG is the Belgian version (The original). You will get a new US reciever, HTS set, barrel, gas piston and p grip typically on either model.
The SA58 is a bastardization of the Rhodesian FAL. The true Rhodie has a 1" longer buttstock to accomodate lanky Swahilis.
To the best of my knowledge nobody builds a reproduction steel lower reciever or bolt carrier/bolt not to mention gas blocks, sights and bipods.
I believe DSA builds an aluminum Para lower but they are pricey.