Subsequent processing for export permitted under Rule 12(1)(b) with Commissioner approval and AR-5 supervision. Subsequent processing of duty-paid goods for export is permissible only under Rule 12(1)(b) with Commissioner approval and specified safeguards: processing at a Commissioner-approved place ('warehouse'), export under proforma AR-5 generated at that place, opening of input packages under excise officer supervision with sampling and testing to verify duty-paid character and composition, declaration of input-output ratio, and defacement of invoice copies. Discrepancies found on verification bar export under Rule 12(1)(b); alternative export routes such as drawback may be available. Prior inconsistent circulars are superseded.