The Holley model 1940 is the first of a series of similar 1 barrel downdraft carburetors, starting with model 1940 in 1970 and proceeding into the 1980s with models 1945, 1946, 6145 and 6146. The 6xxx series are feedback carburetors used in the 1980s.
Many model 1940 carbs were used on Ford industrial engines and as service replacements for Autolite (Ford) 1100 carbs. Model 1945 (and later, 6145) carbs were used extensively on Chrysler Corp Slant 6 engines from 1974 to the mid-1980s.
The carb number is usually stamped on the side of the float bowl. How to find Holley carburetor list numbers