I suppose the easy definition of "spreadbore" is larger rear butterflies then front. But most people I know consider the AFB, AVS, Holley to be squarebore and T-quad and Q jet to be spreadbore carburetors. A LOT of carburetors actually have smaller front than rear butterflies it's obvious on this AFB, but if you take apart as many carbs as I have you'll find the butterflys seldom interchange front to back.