The ’57 Ford dual quad intakes all have those threaded plugs in the bottom.  Those were to facilitate the casting process.  While many of the Ford produced aluminum intakes for their various engine families do have those plugs in the bottom, most of the aftermarket intakes do not employ them.  These just represent differences in how the manifolds are cast.

Lorena, Texas (South of Waco)

I have noticed that the bottom surface of the factory dual quad manifolds sometimes has two large set screw type bolts while some manifolds have a completely smooth bottom surface. What is the significance of these differences? Pictured below is a two-bolt type.

 

Steve