Interest on delayed GST returns: phased nil-interest grace periods followed by applicable interest for specified tax months. Substituted proviso prescribes interest treatment for registered persons filing FORM GSTR-3B: two turnover-based classes receive phased nil-interest grace periods for designated tax months, after which an applicable interest rate applies for the remainder of the specified period. The amendment links these temporal cut-offs and rates to particular tax months and is issued under section 50(1) read with section 148 of the Madhya Pradesh GST Act.