I want to say "card to base" probably had something to do with the capsense boards where they had the soldered wires running from the controller to the PCB. If that's screwed up it took some work to redo.
The stickers got reused for the M, and I'm guessing that test was folded into "merge", which may be the first test after the internals are put together. Way easier to swap out an M controller and put the keyboard back in the line.
Just a guess though.