Retrospective amendment of customs notification enables refunds for duties collected, subject to a six month claim window. The provision deems a retrospective amendment of a customs notification by substituting specified tariff entries, treats the Central Government as having had the retrospective power to amend that notification at all material times, mandates refund of customs duty collected but not payable had the amendment been in force, requires refund claims within six months of the Finance Bill's presidential assent, and confirms applicability of the statute's existing refund procedural regime to such refunds.