This AFP Plug is meant to plug the vacuum port on the rear cylinder for your single carb VT600 Honda Shadow! It's the world famous AFP Plug! It's got threads on it and it's stainless steel so it will outlast anything!
VT600 1999-2007 with Auto Fuel Petcock
VLX600 1999-2007 with Auto Fuel Petcock
If you don't have this on your single carb bike, you have clearly missed the latest fashion performance trend since the underwire for men! Kidding! The AFP was a vacuum operated diaphragm on the single carb Honda Shadow VT600 and acted as a fuel shutoff in case the bike fell over and died. However, the part was recalled by Honda and the fix was merely to bypass from tank, straight to the carbs.
If you have a single carb bike and have removed the Auto Fuel Petcock, (if you haven't, you should, its a recalled part) you are going to be left with a little tiny hose coming out of the rear cylinder head right by where the intake mates to the head. You can unscrew this little brass port with an 8mm open end wrench (aka line wrench) then use one of these nice stainless plugs to make sure you don't have a vacuum leak! Awesome!