Hi Ron,
That just means that all 4 barrels of the carb are pretty much the same bore size. A "spread bore" carb is like a Thermoquad or Quadrajet, where the front, primary barrels are a lot smaller than the rear secondaries. They started that for mileage and emissions in the '70's.
Some older Carter carbs have primary and secondary barrels obviously a "little" different, but are probably still considered "square bore".