Pulling in a bunch of threads here:
1) Gold and copper mounts are not Systems.
The Gold is solid 9K, so 0.375 purity (typically 9K gold is 37.5% gold; 42.5% silver; 20% copper). While some factories have used 18K gold, 9K is a good balance for Peterson, both because pure(r) gold, so not as good for spigots and the like, and very expensive (and, goodness, the gold mounted pipes are already pretty pricey).
Copper has only been used for Christmas pipes, as bands, army mounts and spigots.
2) Yes, as others have pointed out, Peterson uses sterling silver and nickel mounts most of the time. The System Standard range uses nickel; Deluxe System, the older Premier System and System Spigots use sterling silver. Peterson doesn't use aluminum or steel.
3) The System condenser--that metal thing in the end of the stem--appears only on Deluxe System these days. There's an acrylic equivalent as part of the stem on System Standard.
4) The same shapes appear with different designations (for arcane 100-year-old reasons that are (mostly) lost to history) in all three lines. They can look pretty different because the ends of the shanks are turned differently for each of the three series, but the basic bowl shapes are the same. So, for example, the 9s and the 307 are the same basic shape, with the shanks turned differently.
5) Yes, we discontinued the Premier System in 2019. It fit awkwardly between System Standard and Deluxe System by the time we took over Peterson in 2018 as it had evolved over the previous 20 years to be closer to a System Standard, but with a silver mount and a different stain.
We nixed it in favor of expanding the Deluxe System range to include Deluxe System Dark Smooth (with bowls of equivalent quality to the Premier System Smooth) and Deluxe System Sandblasted (for which we reserved particularly nice sandblasts). Later we added Deluxe System PSB for the nicest/deepest sandblasts.
The System Spigot series was created as a limited thing in the mid-2010s in a handful of shapes. We brought it back to better fill that gap between System Standard and Deluxe System in a more appealing way (with vulcanite P-Lips, for example).
Finally, we resurrected the Supreme System--though very few actual pipes (11 total in two years!) have ever been made because bowls of sufficient quality are so scarce--to hold the very best smooth bowls and the very best sandblasted bowls.
I hope that answers some questions (though other people had shared most of this).
Sykes