Requirement of Section 11A show cause notice bars recovery unless procedural formalities are complied with. Department cannot recover differential excise duty unless a show cause notice is issued under Section 11A and the procedural formalities for that notice are complied with; the Supreme Court so found where a demand notice preceded proper classification and subsequent Section 11A procedures. The Board instructs field officers to adhere to these formalities and holds Collectors accountable if revenue is lost due to non-compliance.