Interest on undisclosed income: late block-period returns attract monthly simple interest and possible penalty subject to specified exceptions. Failure to furnish the return of undisclosed income for the block period after search or requisition attracts simple monthly interest on tax computed on such undisclosed income until assessment completion; a penalty equal to fifty per cent of the tax on determined undisclosed income may be imposed, subject to exceptions where return is furnished and tax paid, requirement of reasonable opportunity to be heard, higher-level prior approval for larger penalties, and specified limitation rules excluding rehearing and court stay periods, with mandated transmission of penalty orders to the Assessing Officer.