Mark, we are saying the exact same thing, I just tweaked one portion to make it easier to understand.
Note that the center vac port on the carb air horn is to provide full manifold vacuum to external sourcess....for example, choke breaker on later 2F carbs.
Yes it exists on all 2F carburetors. The vacuum source for the Power Valve is the rifled carb body screw on the base of carburetor. Once....3inHg of vacuum ( I believe) hits the manifold, the Power Piston is pulled away from Power Valve. As vacuum drops, the spring overpowers the reduced vacuum...