Actually,the casting has provisions for mixture screws in both locations depending on year and application.If not used they are plugged.Here is a 70 4spd I recently finished.Note location.I have taken a lot of carbs with the one screw,which incidently is a left hand thread and eliminated it and went with two screws in the base.Much easier to tune.Also,on the carbs with one mixture screw,you will find pre-adjusted mixture screws behind the plugs in the base due to emission requirements.