JD Lawn & Garden Tractor Forum
Here are the 400 series serial number breakdowns.
400 Serial Numbers:
1975 = 30,001 - 55,000 (19.9hp Kohler Engines)
1976 = 55,001 - 70,000 (19.9hp Kohler Engines)
1977 = 70,001 - 80,000 (19.9hp Kohler Engines)
1978 = 80,001 - 95,000 (19.9hp Kohler Engines)
1979 = 95,001...