Persons appointed, employed, or assigned after May 19, 1959, under former section 1787(c) of this title or section 2385(d) of this title for the purpose of performing functions under the Mutual Security Act of 1954 (see Short Title note set out under section 1754 of this title) and the Foreign Assistance Act of 1961 (section 2151 et seq. of this title) outside the United States shall not, unless otherwise agreed by the agency in which such benefits may be exercised, be entitled to the benefits provided for by this section in cases in which their service under the appointment, employment, or assignment exceeds thirty months. See Ex. Ord. No. 12163, § 1–602(span), Sept. 29, 1979, 44 F.R. 56677, as amended, set out as a note under section 2381 of this title.