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Cut-offs mainly take their origins in major period rule changes.
If you take 1965 and 1966 there were drastic changes in the Homologation processes for GT cars but also in other categories. 100ex minimum production, special bodies, etc.
Take Appendix K Period GR for sports cars, 1966 to 1971 englobes the regular changes which made sports car evolve from Group 4 to Group 5 and stops right before the 1972 drop in engine capacities and so on.
1981 is basically the end of Group 1 to 8 and change towards Group A and all.
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