That's a good question, with some simple answers. The bodies are the same as they use the same casting, it's the internals that make the difference. The main jets are completely different for all four, typically California smog carburetors use a smaller jet, the secondary jets are the same except for the non smog auto, the booster venturies are different and the chokes covers are different for manual and automatic transmissions (that's typical though). The adjustable settings are different too, but that is not physical. The 66 Comet, Falcon, Fairlane and Mustang Shop Manual documents this well. If you find the same main jet used on the four different carburetors, it would most likely be due to someone "tinkering".
Jim
Drink some "Yellow Tail", it helps