Encumberance certificate

Dear sir We have a property in chennai purchased in 1971 for a price of 60000/-. There was a loan of Rs 20000/- taken by the seller in 1963 with mortgage of property.The loan amount of Rs.20000/-was paid along with the balance amount Rs.40000/-and it was mentioned in the sale deed that the loan amount has been paid separately to the seller to clear the loan along with interest Rs.2000/- The mortgage deed was to be cancelled by the lender and he did it. It was mentioned in the letter of documents received by the Bank while we got loan for our business by depositing the collateral of this property. However we do not have copy of the letter of cancellation of loan and the manual EC of the year 1972 reflects the loan taken not discharged.Now we want to sell the property but the buyer wants us to get the mortgage cancelled. The seller and their legal heirs are untraceable and the lender of loan is not known to us and we could not trace the whereabouts of him. The matter is 51 years old and the EC for the last 48 years is clear and no encumberance reflected. We want to know if EC is to be necessarily rectified after 50 years since the maximum period of Limitation act 1963 is 30 years. Please advice A D JAIN