that is a serial number. It is another way of saying "on engines of this model made on or after this serial number" then up to the next serial number series. The factory plans to make tooling and parts changes on a certain date. They record the serial number. In WWII the US Army Air Corp drove Ford crazy making constant changes to the B-24 design. Every time the whole assembly line had to be changed. Three changes in a week, 3 assembly line stoppages and changes. Nothing gets done. Since then assembly lines plan changes on a set date so everything after that date until the next change gets all the changes at once. Then there is only one assembly line change. Might use a carb model X up to a certain date then change to Y. If the mount, manifold, linkage etc don't change then either carb would work. Might have been one carb manufacturer gave a better price, or had better quality control, or delivery.