For the purposes of this subchapter—

((a)) ** “Fully insured individual” defined** The term “fully insured individual” means any individual who had not less than—

((1)) one quarter of coverage (whenever acquired) for each calendar year elapsing after 1950 (or, if later, the year in which he attained age 21) and before the year in which he died or (if earlier) the year in which he attained age 62, except that in no case shall an individual be a fully insured individual unless he has at least 6 quarters of coverage; or

((2)) 40 quarters of coverage; or

((3)) in the case of an individual who died before 1951, 6 quarters of coverage;

((b)) ** “Currently insured individual” defined** The term “currently insured individual” means any individual who had not less than six quarters of coverage during the thirteen-quarter period ending with (1) the quarter in which he died, (2) the quarter in which he became entitled to old-age insurance benefits, (3) the quarter in which he became entitled to primary insurance benefits under this subchapter as in effect prior to , or (4) in the case of any individual entitled to disability insurance benefits, the quarter in which he most recently became entitled to disability insurance benefits, not counting as part of such thirteen-quarter period any quarter any part of which was included in a period of disability unless such quarter was a quarter of coverage, and who satisfies the criterion specified in subsection (c).August 28, 19501950-08-28

((c)) ** Criterion described** For purposes of subsections (a) and (b), the criterion specified in this subsection is that the individual, if not a United States citizen or national—

((1)) has been assigned a social security account number that was, at the time of assignment, or at any later time, consistent with the requirements of subclause (I) or (III) of ; or

((2)) at the time any such quarters of coverage are earned—

((A)) is described in subparagraph (B) or (D) of ,

((B)) is lawfully admitted temporarily to the United States for business (in the case of an individual described in such subparagraph (B)) or the performance as a crewman (in the case of an individual described in such subparagraph (D)), and

((C)) the business engaged in or service as a crewman performed is within the scope of the terms of such individual’s admission to the United States.