Deficiency assessments.

(b) Except in the case of a wilfully false or fraudulent return with intent to evade the tax, no assessment of additional tax shall be made after the expiration of more than three years from the date of the filing of a return or from the original due date of a return, whichever is later; provided, where no return has been filed as provided by law, the tax may be assessed at any time. Where, before the expiration of the period prescribed herein for the assessment of an additional tax, a taxpayer has consented in writing that such period may be extended, the amount of such additional tax due may be determined at any time within such extended period. The period so extended may be further extended by subsequent consents in writing before the expiration of the extended period.