26 C.F.R. § 301.6501(j)-1
With respect to taxable years ending after December 31, 1961, a deficiency attributable to the application to the taxpayer of an investment credit carryback may be assessed at any time before the expiration of the period within which a deficiency for the taxable year of the unused investment credit which results in such carryback may be assessed, or, with respect to any portion of an investment credit carryback from a taxable year attributable to a net operating loss or capital loss carryback from a subsequent taxable year, at any time before the expiration of the period within which a deficiency for such subsequent taxable year may be assessed. For purposes of this section a deficiency shall include a deficiency which may be assessed pursuant to the provisions of section 6213(b)(2), but only those arising with respect to applications for tentative carryback adjustments filed after November 2, 1966.
26 C.F.R. §301.6501(j)-1