If the taxpayer omits from the income stated in the return of a tax imposed by this article an amount properly includable therein which is in excess of 25% of the income so stated, the tax may be assessed, or a proceeding in court for the collection of such tax may be begun without assessment, at any time within 6 years after the return was filed.
61 Pa. Code § 119.15
This section cited in 61 Pa. Code § 119.14 (relating to limitations on assessment and collection).