Holley carburetor identification numbers are typically stamped on the side of the airhorn. If the carburetor was factory issued, it may have multiple numbers, including the manufacturer's part number, date and an application code. The only four-digit number is the Holley identification number.
As of April 2015, no other manufacturer uses a four-digit model number. Holley has produced carburetors in various styles for more than 100 years, and some of the models may not have an identification number stamped on them. A visual inspection can determine the model of the carburetor in question. Carburetion.com has illustrations and diagrams of most Holley carburetors to assist users in model identification.