Man, I love that Apple is showing love for their vintage stuff and their heritage! This, the iPads and iPhones having their original wallpapers now as well, the iPods in their marketing, I love it.
For people curiously looking for where these "wallpapers" are in the system, and since basicappleguy couldn't find them, I may know why.
I simply searched for "Macintosh" throughout the entire system and came across: WallpaperMacintoshExtension.appex among other Wallpaper...appex files. Sifting through, it looks like these backgrounds are generated by this little program so we'll never find the "wallpapers". I guess this means more dynamic styles though could be added in the future.
Path is: "/System/Library/ExtensionKit/Extensions/"
This wallpaper is looking great, but as much as I like the Spectrum color mode with its changing color, I am very annoyed that it shows a different color on each screen (running 2 external screens I usually have 3 different colors). That's too colorful for me, I wish all screen would change the color simultaneously.