Is it possible to register a sale deed if the owner has paid all taxes

My parents (Hindu) had purchased a flat in a Cooperative Housing Society in 1984, in their joint names, in Pune. The Sale Deed was perhaps not registered by the Builder at that time (1984). My father has since expired. If my mother plans to sell the apartment in future, will the fact that the original agreement was "Non Registered" pose any legal hurdle? The builder now claims that the agreement was registered in 1984, however, as per them, at that time, the Department of Registration used to give only a receipt and the original registered agreement was to be collected from them after a couple of years. I doubt if the agreement was registered, since no such receipt is available with us. In case my mother wishes to get the original agreement of 1984 registered now, is it possible, & if so, what is the procedure and documentation required? Index 2 is not available, but Property Tax Receipts since 1985 onward are available.