Sig said they do not have a 20" barrel for the 716. They don't know if any aftermarket barrels would work.
I went ahead and asked them about the gen. 1 and gen. 2 and they said there is no such thing with the 716 only the 516 has gen.1 and gen.2 not the 716. There are different equipment changes over time but no differences in the rifle itself that would warrant a gen.1 or gen.2 title through Sig.