Castings were made in different places around the country (or world). The cast parts needed to be poured weeks if not months before the engine was assembled so it is normal to find an engine that has casting dates that are months earlier than the engine production date. Sometimes the casting dates could be a year or more earlier than the engine production date. Detroit in the 70's was not "just in time", it was more like "pile the parts over there and we'll get to them later".