Looking through the other Opus 12 Flat icons created, I found the Directory Opus Basic v2.0 icon set. I was curious to see how the latter compared to those from the official Internal Icon Set, and created two "diff" images to show the differences. Only then did I notice this post... so for those curious about how exactly the icons changed between the 2013 Basic v2.0 set and the 2016 Internal Flat one:
Directory Opus Basic v2.0
-> 34 unique icons outlined in red, 42 early version icons:
Internal Icon Set (Flat)
-> 17 newly-created icons outlined in orange, 42 updated icons:
Note: Personally I prefer the Internal icons and would suggest putting the Directory Opus BASIC Icons v2.0 Delta Drop-In Replacement right after the Internal Set, to keep the 42 shared updated internal icons, but still be able to access the interesting 34 unique earlier Basic v2.0 ones.