If I had to guess, I'd guess they went to aluminum when the price of copper went through the roof. IIRC, copper (price) is back down again, so they may have gone back to using copper.
I'm surprised they managed to get approval for an aluminum bus bar, all things considered:
Even though copper has a long history as the material of choice for conducting electricity, aluminum has certain advantages that make it attractive for specific applications.
Aluminum has 61 percent of the conductivity of copper, but has only
30 percent of the weight of copper. That means that a bare wire of
aluminum weighs half as much as a bare wire of copper that has the same
electrical resistance. Aluminum is generally more inexpensive when
compared to copper conductors.
Aluminum conductors consist of different alloys known as the AA-1350
series and AA-8000 series. AA-1350 has a minimum aluminum content of
99.5 percent. In the 1960s and 1970s, due to the high price of copper
relative to aluminum, this grade of aluminum began to be popularly used
for household wiring. Due to low-quality workmanship at connections and
the physical differences between aluminum and copper, high-resistance
connections formed and became a fire hazard.
As a response, aluminum alloys were developed to have creep and
elongation properties more similar to copper. These AA-8000 series
alloys are the only solid or stranded aluminum conductors permitted to
be used according to Article 310 of the 2014 National Electric Code*.
AA-8000 series alloys meet the requirements of ASTM B800, Standard
Specification for 8000 Series Aluminum Alloy Wire for Electrical
Purposes–Annealed and Intermediate Tempers.
https://www.anixter.com/en_ca/resources/literature/wire-wisdom/copper-vs-aluminum-conductors.htmlBut what do I know, I've not paid much attention to this area of panel tech, other than noting that copper is the way to go - the end.
