(a)(1) If the claim for credit or refund relates to an overpayment of income tax on account of—

(i) The deductibility by the taxpayer, under section 166 or section 832(c), of a debt as a debt which became worthless, or, under section 165(g), of a loss from the worthlessness of a security, or

(ii) The effect that the deductibility of a debt or loss described in subdivision (i) of this subparagraph has on the application to the taxpayer of a carryover, then in lieu of the 3-year period from the time the return was filed in which claim may be filed or credit or refund allowed, as prescribed in section 6511 (a) or (b), the period shall be 7 years from the date prescribed by law for filing the return (determined without regard to any extension of time for filing such return) for the taxable year for which the claim is made or the credit or refund allowed or made.

(2) If the claim for credit or refund relates to an overpayment on account of the effect that the deductibility of a debt or loss, described in subparagraph (1) of this paragraph (a), has on the application to the taxpayer of a net operating loss carryback provided in section 172(b), the period in which claim for credit or refund may be filed shall be whichever of the following two periods expires later:

(i) Seven years from the last date prescribed for filing the return (determined without regard to any extension of time for filing such return) for the taxable year of the net operating loss which results in such carryback, 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 net operating loss which resulted in the carryback.

(3) In the case of a claim for credit or refund involving items described in this section, 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 deductibility of items described in subparagraph (1) of this paragraph (a). If the claim involves an overpayment based not only on the deductibility of items 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 deductibility of items described in subparagraph (1) of this paragraph (a), 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.

(4) If the claim involves an overpayment based not only on the deductibility of items 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.

(b) If a claim for credit or refund is not filed within the applicable period described in paragraph (a) of this section, then credit or refund may be allowed or made only if claim therefor is filed or if such credit or refund is allowed within any 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 the case of credit or refund allowed within such applicable period as prescribed in section 6511 (b) or (c).

(c) The provisions of this section and section 6511(d)(1) do not apply to an overpayment resulting from the deductibility of a debt that became partially worthless during the taxable year, but only to an overpayment resulting from the deductibility of a debt which became entirely worthless during such year.

(d) The provisions of paragraph (a) of this section with regard to an overpayment caused by the deductibility of a bad debt under section 166 or section 832(c), or of a loss from the worthlessness of a security under section 165(g), are likewise applicable to an overpayment caused by the effect that the deductibility of such bad debt or loss has on the application to the taxpayer of a carryover or of a carryback.