Extension for furnishing return permitted where prior-year audit liability and accounts remain unaudited, on written request. Amendments reframe notice scope to refer to total undisclosed income and insert a proviso allowing a limited extension to furnish a return where the prior relevant year's filing due date had not expired before a search or requisition, the assessee was liable for audit for that year, the accounts remained unaudited on notice date, and the assessee requests in writing additional time to get the accounts audited.