Document of sale

Dear Sir, I am willing to purchase a flat from a lady, the lady says that she has no document apart from the share certificate that was issued by the society in the year 1967. The said share certificate was issued by the society in the year 1980 when their society was converted from tenancy to ownership. The then tenants have acted as promoters formed a CHS LTD society and entered into a 999 lease with the landlord. On scrutiny if the said lease documents it was found that the sellers/ladies name is not mentioned in the promoters list. On further inquiry with the CHS we were informed that since the lady and her family weer out of town and her flat remained empty, the then available tenants proceeded with the registration of the 999 year lease document. Shouldn't there be any other document like an agreement to convert the present tenant to owner to justify the sale. Please help since the CHS says there are no other documents other than the share certificate and registered lease deed. How do we justify her ownership link. Please help property is in Mumbai Regards Sanjay