Holley carburetor numbers are referred to as 'List' numbers. They are usually stamped on the carburetor body and preceded by the word 'LIST', the letter 'R' or the letter 'O' for some late models. Numbers that are preceded by '6R' or other combinations of numbers and the letter 'R' are casting numbers referring to a particular part of the carburetor and cannot be used to identify the carburetor. How to find Holley carburetor list numbers