Tariff amendment alters duty computation for aluminium products by substituting an ad valorem rate and an adjusted per tonne allowance. Under rule 8(1) of the Central Excise Rules, 1944, Notification No. 183/84 is amended for tariff item 27: S. No. 6's column (4) is replaced to require duty equal to the appropriate amount chargeable less a specified per metric tonne allowance; S. No. 9's column (4) is replaced to specify an eleven per cent ad valorem duty.