Exemption of maintenance spares prevents recovery of differential customs duty for the retrospective disputed period. The Central Government directed that customs duty and auxiliary duty in excess of the concessional rate under Notification No. 315/83-Cus shall not be required to be paid in respect of maintenance spares imported with goods for initial setting up or substantial expansion of electronic manufacturing units, for the period commencing 27th November, 1983 and ending 1st July, 1992, thereby waiving recovery of differential duty in light of prevailing assessment practice and a subsequent amendment clarifying applicability.