Period of limitation upon assessment and collection.
Section 318
SEC 318. Period of limitation upon assessment and collection.—Except as provided in the succeeding section, intern ai-revenue taxes shall be assessed within five years after the return was filed, and no proceeding in court without assessment for the collection of such taxes shall be begun after the expiration of such period. For the purposes of this section, a return filed before the last day prescribed by law for the filing thereof shall be considered as filed on such last day: Provided, That this limitation shall not apply to eases already investigated prior to the approval of this Code.