When the secondary butterflies exceed past the axis between the front to rear mounting bolts, it can be considered a spread bore. Anything with all the butterflies within that axis is a square bore. Lots of early carbs had larger secondary bores, but were by no means a spread bore.
The Q Jet was the first carb I ever saw in 1967 that exceeded those boundaries of the axis of front to rear mounting bolts. the Thermo Quad was also of that same design. Holley had to make their interpretation of a spread bore too.
AFBs and AVSs were never considered a spread bore carb in any book, or with any of the people I ever knew in the car business, or hobby.
