By your number, your 680 was made in 1984. It is the same as the more popular model 682 but uses the 12 HP Kohler single rather than the 17 HP twin in the 682. These were made by MTD in lesser quantities in response to previous Cub Cadet owners who wanted a replacement for their aging older Cubs yet still wanted a garden tractor with a cast iron single cylinder engine. Smart on their part, as the K301AQS 12 HP single is a much better engine than the original KT17 twins used in the 682/782's.
-Fordlords-