Limitation on service tax assessments: invalid extended notices treated as standard-limit notices enabling assessment for prescribed prior period. Where an appellate authority, tribunal or court finds that a notice issued under the proviso to section 73(1) is not sustainable because charges of fraud, collusion, wilful misstatement, suppression of facts or contravention with intent to evade are not established, the Central Excise Officer shall determine the service tax payable for eighteen months as if the notice were issued under sub section (1) for offences subject to the normal eighteen month limitation.