To the OP, dual plane=2 different levels at carb plenum. With or without divider.
I had an Offy "dual port" where each port had a divider weird looking thing...I never ran it.
Edit here's the "dualport 360" I had.
And a good conversation thread on it from Paul.(he knew his stuff)
Offenhauser Dual-Port 360* Intake ~ 'Was It A Gimmick'
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