HOLLEY CARBURETOR, all Holley carbs are identified by the list number generally located on the front of the choke tower. ex. list 4776-2 is a 600 double pumper. all dual line holley's are not double pumpers, dual line refers to fuel being fed at each end of the carburetor, double pumpers have a accelerator pump on both ends and have mechanical linkage to the secondary (rear) throttle plates. single pumpers have a pump on the front only, some dual line holley's, ex. list 3310-2 are dual line single pumpers that have vacuum operated secodaries, identified by the large vacuum housing on the( rear) secondaries

"google" search  the list number  for fast carb i.d.

don't buy a holley that has had the choke tower milled off for street use and also beware of painted carburetors, assume any used carb will need a rebuild kit installed or at least a jet change and gaskets.and bid accordingly. always use a good filter they are very sensitve to dirt