20 C.F.R. § 233.3
If a person is entitled to a retroactive payment for a month or months in an earlier fiscal year, the reduction factor as imposed with respect to the windfall component of the person's annuity, including that portion attributable to an earlier fiscal year, shall be the reduction factor applicable in the year of payment: Provided, however, That if the application of the payment year reduction factor would result in a larger payment than would the application of the earlier year reduction factor, the earlier year reduction factor shall be applied. The reduction factor imposed in the case of a replacement payment shall be that reduction factor which was applicable to the original payment. The term "replacement payment" means a payment made to a beneficiary to replace a check which was issued to the beneficiary in an earlier month, but which was not negotiated, and "replacement payment" also means a payment made to the beneficiary for an earlier month in which his or her annuity was not paid for some reason such as lack of a current address.
20 C.F.R. § 233.3