26 C.F.R. § 301.7507-10
If, in the case of a bank within section 7507(b), segregated assets (including earnings therefrom), in excess of those necessary for payment of outstanding deposits become available, such excess of segregated assets shall be applied toward satisfaction of accumulated outstanding taxes previously immune under the section, and not barred by the statute of limitations. But see § 301.7507-3 . Where sufficient segregated or unsegregated assets are available, statutory interest shall be collected with the tax. When unsegregated assets or earnings therefrom previously immune become available for tax collection, they will be available only for collection of taxes (including interest and other additions) becoming due after immunity ceases. See the example in paragraph (b) of § 301.7507-5 .
26 C.F.R. §301.7507-10