It is part of the MP books modification to the intake to increase performance and RPM ability with high lift cams by opening up the plenum to both sides of the carb and engine. Like a single plane intake. Consider it a secret way to cheat when a dual plane is required by the racing rules. Or you just wanna hoodwink somebody.
I have also this intake with that mod. Nice mod. Real nice.
Don't be shy of installing a big cam with this one.