Ford had a policy of using up parts from the previous year if they were similar. That's why you have some odd parts on the earliest cars from each year.
My SN is 6T07T104171. That means it is a 6cyl coupe, built at the Meteuchen, NJ plant and was the 4,171st made that year. The build date was 22 Sept. Without a door tag, the only way to determine build date is to guess at the serial numbers.
When you say motor mounts, you must be talking about the V8, since the six didn't change mounting styles in 66. If so, look at your current mount. If it has a stud that goes vertically thru a metal mount, you have the early one. If it has a horizontal bolt that passes thru a sheet metal mount, you have the later one.