The relevant portion of Section 11 A of the Central Excises Act runs as follows: "When any duty of excise has not been levied or paid or has been short levied or short paid or erroneously refunded, a Central Excise Officer may, within six months from the relevant date, serve notice on the person chargeable with the duty which has not been levied or paid or which has been short levied or short paid or to whom the refund has erroneously been made, requiring him to show cause why he should not pay the amount specified in the notice: Provided that where any duty of excise has not been levied or paid or has been short levied or short paid or erroneously refunded by reason of fraud, collusion or any wilful mis statement or suppression of fact, or contravention of any of the provisions of this Act or of the rules made thereunder with intent to evade payment of duty, by such person or his agent, the provisions of this sub section shall have effect, as if for the words "Central Excise Officer", the words "Collector of Central Excise" and for the words "six months", the words "five years" were substituted.