Budget Watch Collecting/Seiko serial numbers

Most Seikos have a relatively simple code--The first letter of the serial number is the last digit of the year, the second is the month, using N for November, D for December. If the production dates of the movement are known, and they were made for less than 10 years this works well. Some movements that were made for a long time are more difficult--The 7009 for instance was made longer than 10 years, so the exact date can be difficult. One clue can be the number of digits--From about 1960 to 1967, Seiko used 7 digit serial numbers. In 1967 and 68, they used both 7 and 6 digit numbers, and from 1969 on, used exclusively 6 digit numbers.