(a) Special period of limitation. (1) If the claim for credit or refund relates to an overpayment of income tax attributable to an investment credit carryback, provided in section 46(b), then in lieu of the 3-year period from the time the return was filed in which the claim may be filed or credit or refund allowed, as prescribed in section 6511 (a) or (b), the period shall be whichever of the following 2 periods expires later:

(i) The period which ends with the expiration of the 15th day of the 40th month (or 39th month, in the case of a corporation) following the end of the taxable year of the unused investment credit which resulted in the carryback (or, with respect to any portion of an investment credit carryback from a taxable year attributable to a net operating loss carryback or a capital loss carryback from a subsequent taxable year, the period which ends with the expiration of the 15th day of the 40th month (or 39th month, in the case of a corporation) following the end of such subsequent taxable year); or

(ii) The period which ends with the expiration of the period prescribed in section 6511(c) within which a claim for credit or refund may be filed with respect to the taxable year of the unused investment credit which resulted in the carryback.

(2) In the case of a claim for credit or refund involving an investment credit carryback described in subparagraph (1) of this paragraph, the amount of the credit or refund may exceed the portion of the tax paid within the period provided in section 6511 (b)(2) or (c), whichever is applicable, to the extent of the amount of the overpayment attributable to the carryback. If the claim involves an overpayment based not only on an investment credit carryback described in subparagraph (1) of this paragraph (a), but based also on other items, the credit or refund cannot exceed the sum of the following:

(i) The amount of the overpayment which is attributable to the investment credit carryback, and

(ii) The balance of such overpayment up to a limit of the portion, if any, of the tax paid within the period provided in section 6511 (b)(2) or (c), or within the period provided in any other applicable provision of law.

(3) If the claim involves an overpayment based not only on an investment credit carryback described in subparagraph (1) of this paragraph (a), but based also on other items, and if the claim with respect to any items is barred by the expiration of any applicable period of limitation, the portion of the overpayment attributable to the items not so barred shall be determined by treating the allowance of such items as the first adjustment to be made in computing such overpayment. If a claim for credit or refund is not filed, and if credit or refund is not allowed, within the period prescribed in this paragraph, then credit or refund may be allowed or made only if claim therefor is filed, or if such credit or refund is allowed, within the period prescribed in section 6511 (a), (b), or (c), whichever is applicable, subject to the provisions thereof limiting the amount of credit or refund in the case of a claim filed, or if no claim was filed, in case of credit or refund allowed, within such applicable period. For the limitations on the allowance of interest for an overpayment where credit or refund is subject to the provisions of this section, see section 6611(f).

(b) Barred overpayments. If the allowance of a credit or refund of an overpayment of tax attributable to an investment credit carryback is otherwise prevented by the operation of any law or rule of law (other than section 7122, relating to compromises), such credit or refund may be allowed or made under the provisions of section 6511(d)(4)(B) if a claim therefor is filed within the period provided by section 6511(d)(4)(A) and paragraph (a) of this section for filing a claim for credit or refund of an overpayment attributable to a carryback. In the case of a claim for credit or refund of an overpayment attributable to a carryback, the determination of any court, including the Tax Court, in any proceeding in which the decision of the court has become final, shall not be conclusive with respect to the investment credit, and the effect of such credit, to the extent that such credit is affected by a carryback which was not in issue in such proceeding.