Customs duty cap limits levy on specified tariff heading, exempting any duty beyond the prescribed ad valorem rate. Notification No. 68/87 Cus (1 3 1987) exempts goods under heading No. 98.06 from so much of the customs duty leviable under the First Schedule to the Customs Tariff Act, 1975 as exceeds the amount calculated at the rate of 100 per cent. ad valorem, relying on powers conferred by sub section (1) of section 25 of the Customs Act, 1962.