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The 4180 is an emissions carb. It has a few little design differences like the vacuum passages are in a little different spots, jets screw in at an angle, and it prevents internal flooding somehow. All the others i dont remember, but i read this from a hot rod article some guy put on the internet here
http://www.swcp.com/~dilts/mustangs/holley_4180/
here is the high quality version that you can actually read
http://www.swcp.com/~dilts/mustangs/holley_4180/holley4180.zip