No lease document of property - mumbai

My Mom bought a property in the 1970's . Although, we have all the resgistered documents of the residential property we do not have the original lease ('sannat') document. The room is in a building which was built by the Government those days called MS buildings and hence it is accepted that the lease document ('sanat') is a very important piece of document.

Kindly advise on how we can get this document.

Regards