MRP-based excise treatment: only statutorily required MRP triggers duty under Section 4A; voluntary MRP excluded. Section 4A's MRP-based excise charging applies only where MRP is statutorily required under the Standards Weights and Measures Act or other law; voluntary affixing of MRP does not invoke Section 4A and such packings must be assessed under the general valuation provision of Section 4. Commissioners may require manufacturers to declare details of non-statutory MRP packings, and Assistant Commissioners should verify applicable exemptions under the Standards Weights and Measures Act.