Air Cleaner Base Questions

[65Valiant310]

So I am looking to get a new air cleaner and base for my edelbrock 1406 4-barrel carb, yet the base I currently have, has a 6 inch opening (fits perfectly).
Why do they say 5 1/8 base on all these websites, yet I actually measure 6 inches wide where the carb gasket to air cleaner base goes?
Does this have something to do with squarebore carbs or what?

Any help would be great.
thanks

 

[rumblefish360]

5-1/8 inside diameter of the gasket? They all say 5-1/8 for normal 4bbl.'s.