Property dispute after 39 years

My grandfather purchased 8 kottah in 1979 december in ranchi out of 57 kottah land of seller after taking permission from the government under urban land ceiling act 1976( we have case number) and sale of deed registered in ranchi and name mutated. and built a bungalow on it ..then the remaining land out of 57 kottah was sold to 10 different people by the seller and all are in peaceful possession . Now after 39 years of peaceful possession when we try to sell our land someone comes with a sale of deed of january 1979 of the whole 57 kottah registered in kolkata... they say we purchased 8 months before you entire 57 kottah but they dont have permission from government under ULCA... we ask were they sleeping from 39 years.. they are doing case on us and entire 57 kottah title suit and not letting us sell... in their registry deed there is no mention of urban land celing act or taking permission from the government....so it was illegal to buy 57 kottah that time ..is their registry valid ?