This paragraph applies irrespective of whether the written evidence of the debt obligation is treated as being the property itself or whether the decedent was engaged in business in the United States at the time of his death. For purposes of this subparagraph and paragraphs (k), (l), and (m) of § 20.2105-1 , a debt obligation on which there are two or more primary obligors shall be apportioned among such obligors, taking into account to the extent appropriate under all the facts and circumstances any choate or inchoate rights of contribution existing among such obligors with respect to the indebtedness. The term "agency or instrumentality," as used in paragraph (a)(7)(ii) of this section does not include a possession of the United States or an agency or instrumentality of a possession. Currency is not a debt obligation for purposes of this subparagraph.
26 C.F.R. §20.2104-1