What's this cotter pin on the barrel adjuster for?
I pulled the cotter pin off and it turns out there's a spring inside the grey knob that pushes it down to the blue frame piece.  That engages the 4 bumps on the grey knob with the 4 dents in the frame (giving 4 index positions per rotation of the knob).



Seems like the cotter pin was meant to be removed all along and maybe only acts to hold the grey knob and spring in place before the barrel adjuster is installed on the frame.  I'm not 100% sure that this is the answer to my question, so wanted to throw this information out there before accepting my own answer.