First, welcome to OCC.........
In answer to your question, the IH built tractors were bulit from 1960-1980, the the 82 Series being the last run.
In 1980, IH sold their Cub Cadet Line to MTD, and formed Cub Cadet Corp (aka - CCC).
MTD/CCC basically used the IH design and components through 1990, when the MTD/CCC designed and used their engineering. That remains today.
If you are a IH purist, then any model before 1980.
If you want IH engineering but not a purist, then a model before 1990.
Hope that answers your question.