Weber Marine Carb on 360 la engine?

I see how they could tap into the manifold vacuum now on the AFB. There is a diagonal passage (viewed from the mixture screw side) from the linkage are up to just above the large manifold vac port. Under the bottom plug near the primary, they must have drilled vertical to connect the passage. If used for pcv, it could have been gummed up and hard to clean. I wonder how MBaird's AFB is drilled since light shows through. Maybe a retrofit drill due to clogging?

I think if you apply air to the large round hole in the bottom between the primaries, then plug the pcv and manifold ports with your fingers, it will push the bottom plug out or you can try holding the carb inverted and fill the pcv passage with carb cleaner. A little soapy water won't hurt anything and will show bubbles where any air makes it through. A mini 12v tire inflator usually has a plastic tappered fitting you could use.