It may be relevant in this connection to refer to Section 11 A of the Act, which pro vides 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 rele vant date, serve notice on the person charge able 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 errone ously been made, requiring him to show cause why he should not pay the amount speci fied 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 misstatement or suppression of facts, 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.