Excise duty exemption limits liability to a fixed ad valorem rate for goods under specified tariff headings. Exempts goods falling under the listed tariff heading numbers of Chapters 84 and 85 from so much of the excise duty specified in the Central Excise Tariff as is in excess of the amount calculated at the rate of 5% ad valorem, relying on the powers under sub section (1) of section 5A of the Central Excises and Salt Act, 1944.