4 Carb Blower Manifold

[406mac]

I found this manifold at a swap meet and was told it was for a "Blown Flathead". I assume the round hole in the bottom is for attachment to the supercharger? What I don't understand is the round duct at the front? If the carbs are drawing air in what purpose does that opening in the front do? I have seen Potvin front mounted superchargers but the carb manifolds did not have the front opening..The manifold could work on any type engine including a flathead if you had the correct intake..... Any ideas?