I have a 1966 mustang with an original build sheet showing the VIN 6T08Axxx and a date code of 20K (October 20, 1965). I removed the starter to see the engine block date, which was 4K24, (Oct 24, 1964). The manifold intake date is 4J23 (Sept 23, 1964). I have read that in the busy production times, sometimes parts were left over and used in later vehicles. What I am wondering is if it is possible that the engine is original, even though the date precedes the car manufacture date by a year? I have assumed the engine was replaced at some time, because the date is so different. Anyone have any knowledge of engines and date codes differing by more than a couple of months of car manufacture? Thank you.