return due date
(7) Adjusted overpayment rate (A) In general The adjusted overpayment rate for any interest accrual period is the overpayment rate in effect under section 6621 for the calendar quarter in which such interest accrual period begins. (B) Interest accrual period For purposes of subparagraph (A), the term “interest accrual period” means the period— (i) beginning on the day after the return due date for any taxable year of the taxpayer, and (ii) ending on the return due date for the following taxable year. For purposes of the preceding sentence, the term “return due date” means the date prescribed for filing the return of the tax imposed by this chapter (determined without regard to extensions).
26 USC § 460(b)(7)
None identified, default scope is assumed to be the parent (subpart B) of this section.