Part #21 is a drain screw, there's one at the bottom of each float bowl on your V2. Just need the right size allen key to unscrew it. .
Be sure to not tighten it 'overkill' when you close it up.
IMO the best way to get all the fuel out is to take the float bowls right off (#19). You might find some crud in there that you can blast with pressurized air.
You can remove the bowls without removing the carbs... Also, you can remove the rubber clutch housing exhaust duct for easier access to the rear float bowl.