Greenhouses cleared from the factory in ready-to-assemble...

Greenhouses cleared from the factory in ready-to-assemble condition fall under the specific tariff entry for greenhouses in ready-to-assemble sets rather than the general entry for plant growth chambers, applying Rule 3(a)'s preference for the most specific description. An amendment extending the normal excise-demand limitation period from one year to two years operates prospectively absent express retrospective effect and cannot revive a demand already time-barred. On that limitation basis, the duty demand, interest and penalty were set aside.