If the Department of Revenue is of the opinion that a taxpayer has failed to file a return, or to include in a return filed, either intentionally or through error, items of taxable income, it may require from the taxpayer a return or supplementary return, under oath, in such form as it shall prescribe, of all the items of income which the taxpayer received during the year for which the return is made, whether or not taxable under the provisions of the applicable tax law. If from a supplementary return, or otherwise, the department finds that any items of taxable income have been omitted from the original return it may require the items so omitted to be disclosed under oath of the taxpayer, and to be added to the original return. Such supplementary return and the correction of the original return shall not relieve the taxpayer from any of the penalties to which the taxpayer may be liable under any provisions of law whether or not the department required a return or a supplementary return under this section. [1957 c.632 § 9 (enacted in lieu of 316.555)]