Are you sure? The (now disk shaped) o-ring (and often the shielding metal washer) can only be fished up out of the well by a tiny hook. Bend one up out of safety wire.
Second from left in this line up of special carb tools, cleaners. View attachment 183947
Some (most?) of the carbs I've worked on only had 2 jets.... the idle jet and the main jet.... pilot sits in the cockpit
To adjust the idle mixture, you turn the idle mixture screw.... or "mix" screw if you will. Most of the Holley's, Rochester's, Marvel Schebler's et al.... didn't have a "pilot" jet, they had idle jets.... and therefor an idle "mix" screw. Just because Mikuni want's to reinvent the wheel, don't make 'em an "industry standard." I'll go with the rest of the world an call it a "mix screw" .... just like I taught from a textbook.
Fwiw... I understand that Mikuni's "pilot screw" is just regulating how much of the "already mixed" mixture flows .... but that flow goes into the venturi to adjust the "mixture" that goes into the cylinder. That makes it a "mix" screw.