In addition to the individuals who are participants in the System under subsection (a) of this section, any individual who was appointed as a Binational Center Grantee and who completed at least 5 years of satisfactory service as such a grantee or under any other appointment under the Foreign Service Act of 1946 may become a participant in the System, and shall receive credit for such service if an appropriate special contribution is made to the Fund in accordance with section
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(d) or (f) of this title.