No casting mark with the Weber 'W' on the side of the float bowl, or Weber on the fuel bowl top cover/choke release covers etc would indicate it's a knock off. The knock offs actually look 'better' due to how bright the grade of cast alloy they're made out of - sort of a blue colour. That is a definite red flag. Some debate over the electric choke cover being black or white but that isn't a 100% guarantee especially with buying a used Weber as someone easily could've bought a replacement from somewhere.

*try cleaning and lubing up the timing advance base plate in the distributor. If it's gummed up it will be laggy with it's advance response.